Move apache-airflow docs to subdirectory

[mik-laj]

Current directory structure:

|-- README.rst
|-- _build
|   |-- docs
|   `-- index.html
|-- _doctrees
|   `-- docs
|-- apache-airflow
|   |-- _api
|   |-- backport-providers.rst
|   |-- best-practices.rst
|   |-- changelog.rst
|   |-- cli-and-env-variables-ref.rst
|   |-- concepts.rst
|   |-- configurations-ref.rst
|   |-- dag-run.rst
|   |-- dag-serialization.rst
|   |-- executor
|   |-- extra-packages-ref.rst
|   |-- faq.rst
|   |-- howto
|   |-- img
|   |-- index.rst
|   |-- installation.rst
|   |-- integration.rst
|   |-- kubernetes.rst
|   |-- license.rst
|   |-- lineage.rst
|   |-- logging-monitoring
|   |-- macros-ref.rst
|   |-- modules_management.rst
|   |-- operators-and-hooks-ref.rst
|   |-- plugins.rst
|   |-- privacy_notice.rst
|   |-- production-deployment.rst
|   |-- project.rst
|   |-- python-api-ref.rst
|   |-- redirects.txt
|   |-- rest-api-ref.rst
|   |-- scheduler.rst
|   |-- security
|   |-- smart-sensor.rst
|   |-- stable-rest-api-ref.rst
|   |-- start.rst
|   |-- static
|   |-- templates
|   |-- timezone.rst
|   |-- tutorial.rst
|   |-- tutorial_taskflow_api.rst
|   |-- ui.rst
|   `-- usage-cli.rst
|-- apache-airflow-providers
|   |-- _api
|   |-- index.rst
|   |-- operators-and-hooks-ref
|   `-- packages-ref.rst
|-- apache-airflow-providers-amazon
|   |-- _api
|   |-- connections
|   |-- img
|   |-- index.rst
|   |-- operators
|   `-- redirects.txt
|-- apache-airflow-providers-apache-cassandra
|   |-- _api
|   |-- connections
|   |-- index.rst
|   |-- operators.rst
|   `-- redirects.txt
|-- apache-airflow-providers-apache-druid
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-apache-hdfs
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-apache-hive
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-apache-kylin
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-apache-livy
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-apache-pig
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-apache-pinot
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-apache-spark
|   |-- _api
|   |-- connections
|   |-- index.rst
|   |-- operators.rst
|   `-- redirects.txt
|-- apache-airflow-providers-apache-sqoop
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-celery
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-cloudant
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-cncf-kubernetes
|   |-- _api
|   |-- connections
|   |-- index.rst
|   |-- operators.rst
|   `-- redirects.txt
|-- apache-airflow-providers-databricks
|   |-- _api
|   |-- index.rst
|   `-- operators.rst
|-- apache-airflow-providers-datadog
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-dingding
|   |-- _api
|   |-- index.rst
|   `-- operators.rst
|-- apache-airflow-providers-discord
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-docker
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-elasticsearch
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-exasol
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-facebook
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-ftp
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-google
|   |-- _api
|   |-- api-auth-backend
|   |-- configurations-ref.rst
|   |-- connections
|   |-- example-dags.rst
|   |-- index.rst
|   |-- logging
|   |-- operators
|   |-- redirects.txt
|   `-- secrets-backends
|-- apache-airflow-providers-grpc
|   |-- _api
|   |-- connections
|   |-- index.rst
|   `-- redirects.txt
|-- apache-airflow-providers-hashicorp
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-http
|   |-- _api
|   |-- index.rst
|   `-- operators.rst
|-- apache-airflow-providers-imap
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-jdbc
|   |-- _api
|   |-- connections
|   |-- index.rst
|   |-- operators.rst
|   `-- redirects.txt
|-- apache-airflow-providers-jenkins
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-jira
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-microsoft-azure
|   |-- _api
|   |-- connections
|   |-- index.rst
|   |-- operators
|   `-- redirects.txt
|-- apache-airflow-providers-microsoft-mssql
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-microsoft-winrm
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-mongo
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-mysql
|   |-- _api
|   |-- connections
|   |-- index.rst
|   |-- operators.rst
|   `-- redirects.txt
|-- apache-airflow-providers-odbc
|   |-- _api
|   |-- connections
|   |-- index.rst
|   `-- redirects.txt
|-- apache-airflow-providers-openfaas
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-opsgenie
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-oracle
|   |-- _api
|   |-- connections
|   `-- index.rst
|-- apache-airflow-providers-pagerduty
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-papermill
|   |-- _api
|   |-- index.rst
|   `-- operators.rst
|-- apache-airflow-providers-plexus
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-postgres
|   |-- _api
|   |-- connections
|   |-- index.rst
|   `-- redirects.txt
|-- apache-airflow-providers-presto
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-qubole
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-redis
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-salesforce
|   |-- _api
|   |-- connections
|   |-- index.rst
|   `-- redirects.txt
|-- apache-airflow-providers-samba
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-segment
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-sendgrid
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-sftp
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-singularity
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-slack
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-snowflake
|   |-- _api
|   |-- index.rst
|   `-- operators
|-- apache-airflow-providers-sqlite
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-ssh
|   |-- _api
|   |-- connections
|   `-- index.rst
|-- apache-airflow-providers-vertica
|   |-- _api
|   `-- index.rst
|-- apache-airflow-providers-yandex
|   |-- _api
|   |-- connections
|   |-- index.rst
|   `-- operators.rst
|-- apache-airflow-providers-zendesk
|   |-- _api
|   `-- index.rst
|-- build_docs.py
|-- conf.py
|-- exts
|   |-- __init__.py
|   |-- airflow_intersphinx.py
|   |-- docroles.py
|   |-- docs_build
|   |-- exampleinclude.py
|   |-- operators_and_hooks_ref-transfers.rst.jinja2
|   |-- operators_and_hooks_ref.py
|   |-- operators_and_hooks_ref.rst.jinja2
|   |-- provider_init_hack.py
|   |-- provider_yaml_utils.py
|   |-- providers_packages_ref.py
|   |-- redirects.py
|   |-- removemarktransform.py
|   `-- sphinx_script_update.py
|-- list-roles.sh
|-- rtd-deprecation
|   |-- 404.html
|   |-- conf.py
|   `-- index.rst
|-- spelling_wordlist.txt
`-- start_doc_server.sh

Also close: https://github.com/apache/airflow/pull/12546/files

---

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Read the Pull Request Guidelines for more information.
In case of fundamental code change, Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in UPDATING.md.

[ashb]

Do we have/need a redirect from the old folder name, or is this just for building and not relevant for published docs?

[mik-laj]

@ashb It's just about building. I am trying to handle redirections in a other ticket.
See: apache/airflow-site#318

[github-actions]

The PR is likely ready to be merged. No tests are needed as no important environment files, nor python files were modified by it. However, committers might decide that full test matrix is needed and add the 'full tests needed' label. Then you should rebase it to the latest master or amend the last commit of the PR, and push it with --force-with-lease.