Modernize the Xanadu Sphinx Theme

[Mandrenkov]

This PR updates the existing Xanadu Sphinx Theme to reflect the aesthetics and customization that is currently implemented, to various degrees, by other Xanadu OSS projects, including:

• FlamingPy - Adopt the Xanadu Sphinx Theme flamingpy#17
• Jet - Adopt the Xanadu Sphinx Theme jet#73
• Lightning Qubit - Adopt the centralized Xanadu Sphinx Theme PennyLaneAI/pennylane-lightning#287
• Mr Mustard - Adopt the Xanadu Sphinx Theme MrMustard#126
• PennyLane - Adopt the centralized Xanadu Sphinx Theme PennyLaneAI/pennylane#2450
• Strawberry Fields - Adopt the Xanadu Sphinx Theme strawberryfields#701
• The Walrus - Adopt the Xanadu Sphinx Theme thewalrus#341
• XIR - Adopt the Xanadu Sphinx Theme xir#20
• Xanadu Cloud Client - Adopt the Xanadu Sphinx Theme xanadu-cloud-client#24

A comprehensive .github/ directory is also included in this PR. Once merged, v0.1.0 of the XST will be published to PyPI.

Note to Reviewers:

Clearly, this is a huge PR with many changes; however, most of these modifications have already been indirectly approved through the PRs linked above. I would suggest taking a closer look at the files inside .github/ as well as the root directory; the xanadu-sphinx-theme/ directory is also important, but that review can be limited to cosmetics and style.

[Mandrenkov]

This PR also addresses [sc-18723].

[josh146]

This is amazing work @Mandrenkov 💯

I have one minor question: how do you configure the theme to look like the QML/photonics website, rather than the docs? E.g., hidden left navbar, maybe differently centered pages, different shadows?

[Mandrenkov]

This is amazing work @Mandrenkov 💯

Thanks! Your help was greatly appreciated!

I have one minor question: how do you configure the theme to look like the QML/photonics website, rather than the docs? E.g., hidden left navbar, maybe differently centered pages, different shadows?

I'm not sure about the latter two (perhaps including a local CSS file would work), but I know the first one can be configured by setting html_sidebars appropriately.

[josh146]

I'm not sure about the latter two (perhaps including a local CSS file would work), but I know the first one can be configured by setting html_sidebars appropriately.

Oh does this mean that the Xanadu Sphinx theme can't be used for the QML/Photonics repo, since those themes deviate too much?

[Mandrenkov]

Oh does this mean that the Xanadu Sphinx theme can't be used for the QML/Photonics repo, since those themes deviate too much?

Hmm, I think it is still possible to reuse the Xanadu Sphinx Theme for those websites. Sure, some modifications will be necessary, but I don't think the XST is fundamentally incompatible with those projects.