Adopt the Xanadu Sphinx Theme

[Mandrenkov]

Context:

Currently, all Xanadu OSS projects include their own version of the Xanadu Sphinx theme; however, the Xanadu Sphinx Theme repository is now publicly available and is the preferred way to access the Xanadu CSS theme and Sphinx directives.

Description of the Change:

• Deleted the docs/xanadu_theme directory.
• Updated docs/requirements.txt and docs/conf.py to reference and use the (centralized) Xanadu Sphinx Theme.
• Added a Python 3.10 classifier to setup.py.

Benefits:

• The size of this repository is smaller, meaning git clone will run faster.
• Updates to the Xanadu Sphinx theme no longer have to be manually ported to this repository.

Possible Drawbacks:
None.

Related GitHub Issues:
Depends on XanaduAI/xanadu-sphinx-theme#2.

[thisac]

Cloned and built the docs, and it seems to work! The content side-bar didn't seem to scale correctly when shrinking the screen with. Other than that, everything looked great! 💯

[Mandrenkov]

Cloned and built the docs, and it seems to work!

Awesome, thanks for taking the time to build this locally, @thisac!

The content side-bar didn't seem to scale correctly when shrinking the screen with.

Great catch! 🎣 This issue has now been addressed by XanaduAI/xanadu-sphinx-theme@1e3b663. So, if you re-install the Xanadu Sphinx Theme, you should see that the local ToC is properly hidden for narrow viewports.