Add Sphinx documentation

[Mandrenkov]

Context:
It is preferable to have Sphinx documentation available for the XCC in time for the initial public release.

Description of the Change:

• Added Sphinx documentation to the docs/ directory.
• Fixed several docstring examples and added Sphinx capabilities to the cached_property decorator.
• Created a GitHub Actions workflow for building the Sphinx documentation.

Benefits:

• The XCC documentation is more accessible and consistent with other Xanadu open-source projects.

Possible Drawbacks:
None.

Related GitHub Issues:
None.

[josh146]

Looks great @Mandrenkov, thanks for putting this together! I left one ver minor comment, re: including the readme in the documentation, but apart from that 💯

[thisac]

Looks great! Really like the dev guide. It's super-clear! 💯

Not really directly related to this PR, but the settings docs look a bit messy. Not sure how public that needs to be, or if the docstrings can be expanded a bit for it to look better.

.

[Mandrenkov]

Thank you very much for reviews and suggestions, @josh146 and @thisac!

I've updated the PR accordingly (and added docstrings to the Settings class attributes too).

[josh146]

@Mandrenkov this PR is 💯 Above and beyond what I was expected for bare-boned docs!