New pkgdown config

[VisruthSK]

Changed site theme to new one, swapped to BRMS SVG logo, Stan blue hex SVG.

@paul-buerkner feel free to ignore + reject any workflows until PR is ready for review, I'll tag you then!

Fixes #1827.

[VisruthSK]

@paul-buerkner I'm running into an issue when building a vignette on the GHA runner which I don't understand, would you mind taking a look?

 ── Building articles ───────────────────────────────────────────────────────────
Writing `articles/index.html`
Reading vignettes/brms_customfamilies.Rmd
Error: 
! in callr subprocess.
Caused by error in `.f(.x[[i]], ...)`:
! Failed to render 'vignettes/brms_customfamilies.Rmd'.
✖ Quitting from brms_customfamilies.Rmd:88-91 [fit1]
Caused by error:
! invalid connection

https://github.com/VisruthSK/brms/actions/runs/19951704590/job/57212764476

[paul-buerkner]

No clue, but I will check once I have time. Anyone seen this before?