The file tree must match that of the fallback theme (material).
The previous templates were never picked up.
The mkdocs templates folder is simply removed:
every template falls back on the material ones.

The templates for the readthedocs theme however
are updated: they must match the actual theme.

This commit also improves rendering a bit with CSS tweaks.
Still lots of rendering issues, notably with components
from pymdownx (tabs for example), for which these two themes
have no specific styling.