Convert README to quarto from Markdown

[apulsipher]

quarto is preferred over RMarkdown for README file. Convert to using quarto instead.

But be careful to check how it handles badges. quarto can be weird about generating the badges, especially the the cranlogs badge. For this badge, the URLs do not have an extension, but quarto tends to add them (or an older version of it, maybe). Look at this when converting.

[apulsipher]

I think we should wait to convert to quarto until usethis and devtools support it properly. Keeping with .Rmd allows us to use the usethis::use_readme_rmd and devtools::build_readme functions along with the readme-rmd-rendered pre-commit checks and the additional pre-commit hooks installed by usethis::use_readme_rmd.

There is an open PR for creating a use_readme_qmd function on usethis here r-lib/usethis#2176 and an open issue for adding support for building quarto READMEs to devtools here: r-lib/devtools#2620

I suggest we wait to convert until those features are added. Using RMarkdown has no immediately problems, so keeping as much support as we can from other dev tools would be good.