Add documentation page

[trangdata]

I think a brief documentation page of the core functions of pmlb would be helpful.

[JDRomano2]

Ideally, we'd have API docs that automatically compile from the code's docstrings every time the docs pages update. This is a bit nontrivial without migrating the entire documentation to Sphinx (or equivalent) with auto-doc capabilities.

In the meantime, I'm building a static API Docs page that will suffice for now. PMLB only has a few user-facing functions, so this really isn't a bad compromise for the time being.

[trangdata]

I agree. I don't envision the pmlb functions changing that much in the future. Most changes would be coming from existing and new datasets. Static Docs suffice for now.

[lacava]

there will probably be a few new functions added in the next couple months, but other than that I agree.