Fix mkdocs

[etiennebacher]

This does two things:

• do not remove the .md extension anymore in mkdocs.yaml, otherwise the files do not appear on the website (tested with polars)
• simplify the cleanup at the end: basically we can remove docs altogether and rename site to docs

@vincentarelbundock could you test with marginaleffects to see if this breaks anything?

[vincentarelbundock]

Haha, turns out I had to switch the marginaleffects website back to quarto for features like tabset are hard to replicate. Turns out there are advantages to going directly from qmd to html, rather than through md.

So I don't have a complex website to test on now.

[etiennebacher]

I guess content-tabs don't work when vignettes are rendered by R CMD check?

I merge this now since it's a blocker for the rest of the PR in polars

[vincentarelbundock]

Yeah, that was my original plan, but I couldnt easily find a way for Quarto to execute a code block and format it in the exact format that MkDocs requires. I'm sure there's a way but it is beyond me at the moment.