Doc - use sphinx-multiproject for RTD subprojects

[PrimozGodec]

Issue

Orange3 documentation consists of multiple sphinx projects, which are built as subprojects at the Read the Docs. The documentation each subproject builds is set in advanced settings at RTD. That prevents us from using the .readthedocs.yaml file to set a higher version of Python since .readthedocs.yaml overwrites all advanced settings.
We need to change the version of Python used to build documentation (the default is 3.7) since we cannot increase some package versions in Orange.

Description of changes

In order to use .readthedocs.yaml to set a different version of Python, we need to set the documentation project somewhere outside the advanced settings in RTD. Using sphinx-multiproject enables us to set it as an environmental variable which does not get overwritten by .readthedocs.yaml.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #6197 (d9588dc) into master (5b67ff8) will decrease coverage by 0.00%.
The diff coverage is n/a.

❗ Current head d9588dc differs from pull request most recent head 55b4b0e. Consider uploading reports for the commit 55b4b0e to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6197      +/-   ##
==========================================
- Coverage   86.65%   86.65%   -0.01%     
==========================================
  Files         315      316       +1     
  Lines       67976    67991      +15     
==========================================
+ Hits        58905    58917      +12     
- Misses       9071     9074       +3