Update pkgdown.yaml

[VisruthSK]

Clean up the pages branch before deploying the site.

Waiting for https://github.com/stan-dev/loo/actions/runs/19970666932 to finish and ensure the website is still up after.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.78%. Comparing base (30d6677) to head (181db28).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #318   +/-   ##
=======================================
  Coverage   92.78%   92.78%           
=======================================
  Files          31       31           
  Lines        2992     2992           
=======================================
  Hits         2776     2776           
  Misses        216      216           

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[jgabry]

Looks like mc-stan.org/loo is down now but the dev site is still up

[VisruthSK]

I think clean: true will always wipe the main site whenever a dev site is rebuilt because it wipes the entire branch before copying over the (dev site) files. For hygeine, we could manually wipe the pages branch, build the release site, then build the dev site but that will accumulate cruft which may not be cleaned by pkgdown builds (which only clean pkgdown managed files. I'm going to close the PR and try to think about how to maintain the dev site and release site files only.