Incorporate the recommended CSS style in the package

[pawamoy]

Is your feature request related to a problem? Please describe.
It's a bit annoying to copy over the recommended CSS in every project. Also asked here: #159 (review)

Describe the solution you'd like
We can package the recommended stylesheets.
The question is: how to use them in the MkDocs extra_css option.

[oprypin]

I think I'm gonna withdraw this request. I'm also giving users the freedom to put whatever styles they want, and it also works OK without any styles

especially after the removal of .hidden-toc in https://github.com/pawamoy/mkdocstrings/pull/209/files

[pawamoy]

Noted. Could still be nice to have an opt-in bundled CSS file 🙂

[oprypin]

FWIW I know how a plugin could add its files into the site.

on_files: add a file with a chosen arbitrary name.
on_config: insert that file name into extra_css.