mkdocs: support extra_css & extra_javascript

[casperdcl]

Is your feature request related to a problem? Please describe.

mkdocs supports extra config, e.g.:

mkdocs_config:
  extra_css: [somefile.css, somedir/another.css]
  extra_javascript: [js/more.js]

but pydoc-markdown doesn't copy those files over.

Describe the solution you'd like

Automatically pick up and copy these files.

Or at least document a work-around.

Describe alternatives you've considered

work-around:

hooks:
  post-render:
  - cp -r somefile.css somedir/ js/ build/docs/content/