Docs: Clean up reference docs

[lwasser]

This is a start at the documentation cleanup. NOTE: i do need to go through and remove / update methods / classes etc that we've renamed / updated / removed. I've done some of them but not comprehensively. There are also some docstring typos that i need to clean up and missing docstrings.

A few notes

• I had originaly wanted to use sphinx-autoapi. This tools is really nice as it automates many parts of api generation and i think would allow us to remove particular classes from the docs via custom templates. However, i was strugglin to get it to play nicely with autodoc_pydantic.
• So i went back to autodoc but with autodoc if we are automatically generating those api stub files, we can't use the :exclude-members: feature because it just creates the .rst stub file on the fly. i'd have to manually create those which defeats the purpose of using autodoc!

I am going to open an issue in sphinx-autoapi because it's possible i'm missing something with the pydantic piece!

for the time being, this adds the banner at the top of our docs with a note about and link to the new features page for stravalib 2.0. it's not populated at all yet!

it also adds structural organization to the model.py api page to make it easier to parse.

TODOs:

• cleanup / lint / fix docstrings throughout.
• Double check classes and methods align with what we actually have in our API now. If a class was removed or ranamed sphinx is quiet about not rendering it.
• there are a handful of errors i need to clean up as well.

i'm opening this as a draft as there's a bit more work here to do than i expected!