Refactored `build_readme()`

[VisruthSK]

Added support for Quarto documents, deprecated build_rmd(), updated tests. Tests rely on usethis::use_readme_qmdproposed in r-lib/usethis#2176--leaving as draft PR until that gets merged.

Tried to address #2628 by using the regex /(inst/)?readme[.](r|q)md$ to at least ground line ends.

Fixes #2620.