Added multiverse and stan universe to pkgdown GHA

[VisruthSK]

#399 added cmdstanr to GHA but failed to account for the pkgdown GHA. This PR rectifies that oversight.

[jgabry]

Thanks, good to merge when everything passes

[VisruthSK]

pkgdown run

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.13%. Comparing base (183fd49) to head (a9f7562).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #536      +/-   ##
==========================================
- Coverage   99.16%   99.13%   -0.04%     
==========================================
  Files          35       35              
  Lines        5903     5903              
==========================================
- Hits         5854     5852       -2     
- Misses         49       51       +2     

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