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.

[VisruthSK]

In _pkgdown.yml, 2 topics missing from index: "logit" and "pp_validate".
Either add to the reference index, or use @keywords internal to drop from
the index.

Where should these two reference pages go? They're both exported so I presume the documentation should be published. @jgabry

[jgabry]

I think "logit" should go in the "Additional documentation" category in _pkgdown.yml. For pp_validate I think we should eventually deprecate it (the method it uses is outdated), but for now @keywords internal is OK.

[jgabry]

(also don't worry about r cmd check failing if it's unrelated to this PR, that's been an issue with rstanarm on GHA)

[VisruthSK]

I think this PR only needs a redocument and it should be good. I'm trying to do so now.

[VisruthSK]

@jgabry

Accidentally built straight to root w/o a dev site, should I revert or just roll with it and fix the logo/favicons? Same question for rstantools

https://mc-stan.org/rstanarm/
https://mc-stan.org/rstantools/

[jgabry]

At this point I think it's fine to roll with it and fix what's necessary, thanks!

[VisruthSK]

I think I may have changed line endings but I reverted any formatting issues

[jgabry]

Ok thanks

[VisruthSK]

Also doesn't have a dev site but otherwise seems ok.

[VisruthSK]

Math seems to be fixed at https://mc-stan.org/rstanarm/articles/lm.html, rerunning the build after your changes. Was there anything else that needed to be fixed?

[jgabry]

I think that’s it, thanks. I’ll take another look later just to make sure there’s nothing else and if not then I’ll merge it.

[jgabry]

I added _pkgdown.yml to .Rbuildignore to avoid an R cmd check NOTE but otherwise I think this is ready to go.

[jgabry]

Thanks again @VisruthSK!