Better MkDocs support on Read the Docs

[NiklasRosenstein]

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

• Read the Docs injects some custom values into the MkDocs configuration (mostly extra CSS and JS), and Pydoc-Markdown should pick that up.
• Pydoc-Markdown should also pick up the --site-dir argument which is where Read the Docs expects the HTML content to go to. The current workaround is to manually pass --site-dir $PWD/_build/html as an argument to Pydoc-Markdown.

Describe the solution you'd like

Read the Docs kicks of MkDocs builds like this:

$ /home/docs/.../bin/python -m mkdocs build --clean --site-dir _build/html --config-file mkdocs.yml 

And with readthedocs-custom-steps, we still have access to all of these arguments starting with mkdocs. Pydoc-Markdown could have a mode where arguments are passed to the renderer directly, for example

$ pydoc-markdown -- "$@"

Which can then parse the --site-dir as well as the --config-file.