support pretty URL pages

[eitsupi]

Many static site generators support the use of pretty URLs. Currently .html is assumed to come at the end of URL, but it would be great if we could specify / instead.

https://github.com/r-lib/downlit/blob/c0a8f645e21a03e258b7c1684901f84279cc706a/R/link.R#L213

For example, the reference pages of the polars package created by Mkdocs Material are pretty URLs, so downlit cannot create links to the reference pages. https://rpolars.github.io/reference/pl_pl/ https://github.com/pola-rs/r-polars/blob/67e6097b5d8395e32ee589a1dfd756138128c342/man/pl_pl.Rd

Perhaps pkgdown.yml needs to be extended to indicate if pretty URLs are used.

Related to r-lib/pkgdown#2173