docs: initial skafolding

[aarnphm]

This PR scaffolds the initial Sphinx doc structure for prometheus_client

This is a part of #848 in an attempt to better improve prometheus_client docstring.

[csmarchbanks]

Thanks for this PR and the other one, just wanted to say I do see it but am very busy this week. Hopefully I can review them sometime next week!

[aarnphm]

No worries. Let me know if I can help.

[marctc]

I thought we wanted to try mkdocs, but I'm fine with this. I'll try to run it locally to see how it renders :+1:

[marctc]

Hey @aarnphm, I've been trying to run this, but I'm not able to do it, this what I noticed:

• I had to install another package (sphinx-click) that was not in the requirements.
• I'm still not sure how I have to run sphinx and where. Could we add make serve-docs and make generate-docs (or something like that) in the root Makefile?
• I'm running sphinx-autobuild docs/source docs/build/html and I'm getting this warning:

WARNING: html_static_path entry '_static' does not exist

And this error (even manually installing the package)

Exception occurred:
  File "/home/marctc/.local/lib/python3.10/site-packages/markdown_it/rules_core/linkify.py", line 30, in linkify
    raise ModuleNotFoundError("Linkify enabled but not installed.")
ModuleNotFoundError: Linkify enabled but not installed.

The webserver is running but I get 404 if I go to http://127.0.0.1:8000

[aarnphm]

Sorry for the late reponse, I have been caught up with work.

I forgot to specify the sphinx requirements. Should I make a docs-requirements.txt?

[csmarchbanks]

Maybe just add any additional requirements to docs/requirements.txt?