Add PR pkgdown workflow

[VisruthSK]

Change the pkgdown workflow to support building a PR-specific site which is kept up to date on PRs with the "build-website" label.

https://mc-stan.org/loo/pr/403/

See https://mc-stan.org/cmdstanr/pr/1296/, stan-dev/cmdstanr#1306.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.50%. Comparing base (87024b1) to head (6af0c99).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #403   +/-   ##
=======================================
  Coverage   92.50%   92.50%           
=======================================
  Files          31       31           
  Lines        3107     3107           
=======================================
  Hits         2874     2874           
  Misses        233      233           

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[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 6af0c99 is merged into master:

• ✔️loo_function: 1.84s -> 1.84s [-0.27%, +0.01%]
• ✔️loo_matrix: 1.68s -> 1.68s [-0.31%, +0.16%]
  Further explanation regarding interpretation and methodology can be found in the documentation.