chore: Switch from edx-sphinx-theme to sphinx-book-theme

[xitij2000]

The edx-sphinx theme is being deprecated, and replaced with sphinx-book-theme. This removes references to the deprecated theme and replaces them with the new standard theme for the platform.

Testing instructions:

• Run make docs and see that the docs are generated properly and use the sphinx-book-theme

See openedx/public-engineering#200

Reviewers:

• tag reviewer
• tag reviewer

Merge checklist:

• All reviewers approved
• CI build is green
• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Commits are squashed

Post merge:

• Create a tag
• Check new version is pushed to PyPI after tag-triggered build is
  finished.
• Delete working branch (if not needed anymore)

Author concerns: List any concerns about this PR - inelegant
solutions, hacks, quick-and-dirty implementations, concerns about
migrations, etc.

[openedx-webhooks]

Thanks for the pull request, @xitij2000! Please note that it may take us up to several weeks or months to complete a review and merge your PR.

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• supporting documentation
• Open edX discussion forum threads
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Please let us know once your PR is ready for our review and all tests are green.

[mphilbrick211]

Hi @openedx/vanguards! Would someone be able to review/merge this? Thanks!

[openedx-webhooks]

@xitij2000 🎉 Your pull request was merged! Please take a moment to answer a two question survey so we can improve your experience in the future.