[WIP] Docs using Sphinx and Doxygen

[akhiltulluri]

This is still WIP but provides a baseline setup for publishing docs with Sphinx, doxygen, breathe and readthedocs. Related to #127

Preview sample doc generated: https://libcachesimtest.readthedocs.io/en/latest/

[akhiltulluri]

@1a1a11a - do I migrate everything under doc/ into using sphinx and reStructuredText?

Also with doxygen, what are the things that need to be in docs? I think with breathe we've to selectively choose what parts of the docs from code must be shown in sphinx and where.

[haochengxia]

Thank you! Looks good overall.

Let's use env.yml, similar to https://github.com/microsoft/LightGBM/tree/master/docs, or directly use pyproject.toml to manage Python dependencies, which follow a relatively modern style.

[1a1a11a]

You can make the decision on what to include in the doc website. @haochengxia and I will help you iterate on this.

[1a1a11a]

@haochengxia do you think we should have a separate doc for the Python binding or mix with the main doc?

[haochengxia]

We could separate them to avoid overwhelming beginners who want to try out Python packages first.