Fix navbars on dev sites after #11

[jgabry]

See #10 (comment)

Ideally this would be fixed by not having each package define its own navbar.

[VisruthSK]

Fixes merged in loo and cmdstanr.

Open PRs:
stan-dev/priorsense#147
stan-dev/bayesplot#562
stan-dev/rstan#1195
stan-dev/rstanarm#659
stan-dev/rstantools#162
stan-dev/projpred#570
stan-dev/shinystan#211
stan-dev/posterior#466

[VisruthSK]

Some PRs don't seem to have picked up the latest version of the template package and should be re-run after a cache clear to ensure that isn't a real issue. Those PRs have been marked as drafts and I'll take care of them later.