New pkgdown theme

[VisruthSK]

Updating the theme to better match the main Stan page. This PR uses the new theme & use a GitHub Action to automatically build the dev version of the site on PRs and the release version on releases.

Also bumped Roxygen version.

[jgabry]

Once we merge this do you think it makes sense to clean up the gh-pages branch? Right now it's a bit of a mess, since it has an old version of the R package on that branch (because we needed it to build the site the old way). I suppose it doesn't really affect the website, but maybe not ideal? I guess same question for the other packages that were using gh-pages (loo, bayesplot, rstanarm, shinystan).

[VisruthSK]

Yeah I think it makes sense to nuke it and reinit everywhere

[VisruthSK]

Currently the main site has the red logo while the dev site has the main logo. This will be fixed on the next release when the main site is rebuilt, but we could regenerate the main site once.

[VisruthSK]

@jgabry Actually about cleaning up old files, I think setting clean: true in the GitHub Pages deployment step would suffice? I'll test it on pkgdownconfig to see if it works, and will change it separately.

EDIT: looks like that is all that's needed. I think I set it to false when we were still having packages build the site into the docs dir so the whole package doesn't get deleted. Should I go around enabling that before merging or do in a separate PR?

[jgabry]

EDIT: looks like that is all that's needed. I think I set it to false when we were still having packages build the site into the docs dir so the whole package doesn't get deleted. Should I go around enabling that before merging or do in a separate PR?

OK great, glad it's that simple. Yeah might as well add it to these PRs that haven't been merged yet and I guess for the ones already merged (posterior, bayesplot, loo, cmdstanr, any others I'm forgetting?) do a separate PR. Thank you!