Update ReadTheDocs builds configuration

[MisRob]

Summary

• Starting on September 25, ReadTheDocs builds without configuration file won't work anymore. This adds the configuration file.
• Enables sphinx-rtd-theme for the production build
  • Previously, it was enabled only for local builds, probably because it was applied by default by ReadTheDocs. However, ReadTheDocs is generally moving towards requiring explicit configuration and having the theme disabled for production seems to be the cause of some recent troubles with theme on https://kolibri-dev.readthedocs.io/en/develop/

References

https://blog.readthedocs.com/migrate-configuration-v2/

Reviewer guidance

• Preview the configuration file
• Preview the production docs build: https://kolibri-dev--11179.org.readthedocs.build/en/11179/
• Preview the local docs build

Testing checklist

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

PR process

• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
• If PR is ready for review, a reviewer has been added. (Don't use 'Assignees')
• If this is an important user-facing change, PR or related issue has a 'changelog' label
• If this includes an internal dependency change, a link to the diff is provided

Reviewer checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.16.0b5.dev0_git.134.g2bab95dd.pex
Windows Installer (EXE)	kolibri-0.16.0b5.dev0+git.134.g2bab95dd-unsigned.exe
Debian Package	kolibri_0.16.0b5.dev0+git.134.g2bab95dd-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.16.0b5.dev0+git.134.g2bab95dd-0.3.0.dmg
Android Package (APK)	kolibri-0.16.0b5.dev0+git.134.g2bab95dd-0.1.0-debug.apk
TAR file	kolibri-0.16.0b5.dev0+git.134.g2bab95dd.tar.gz
WHL file	kolibri-0.16.0b5.dev0+git.134.g2bab95dd-py2.py3-none-any.whl