Reorganize workflow files

[haasad]

I'm tidying up the pipelines as part of #1094. This PR is mostly a cosmetic change, everything related to the github action pipelines is moved from the ci/ directory in the repo root to .github/ to have it all in one place.

The only exception is the conda recipe for the "stable" AB version which I moved to recipe/meta.yaml. Even if we don't actively use the stable AB version on the bsteubing channel anymore, this recipe is still the place where all dependencies and version pins are configured.

Checklist

• Keep pull requests small so they can be easily reviewed.
• Categorize the PR by setting a good title and adding one of the labels:
  bug, feature, ui, change, documentation, breaking, ci
  as they show up in the changelog
• Link this PR to related issues by using closing keywords.
• Request a review from another developer.

[coveralls]

coverage: 50.066%. remained the same
when pulling e54e645 on reorganize-workflow-files
into 7eebd42 on master.

[github-actions]

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