feat: Use bslib's navbar Sass variables, if present

[gadenbuie]

bslib patches Bootstrap to add three Sass variables: $navbar-bg, $navbar-light-bg and $navbar-dark-bg (see e.g. bslib's Boostrap 5 _variables.scss file).

This PR updates pkgdown's Sass so that $pkgdown-navbar-bg and $pkgdown-navbar-bg-dark inherit the (default) values from bslib so that the same Sass variables can be used in Shiny, Quarto and pkgdown.

This change shouldn't affect anyone who is already using the pkgdown Sass variables as these will take precedence over bslib's navbar bg vars.

Fixes #2703

[jayhesselberth]

Could you add a news bullet? THanks

[gadenbuie]

Could you add a news bullet? THanks

Sure! I also updated the documentation in the customise article to

1. Inform that navbar.bg and navbar.type only take effect when template.light-switch: false.
2. Describe the available Sass variables for selecting navbar background colors when template.light-switch: true.

[remlapmot]

This is great - this pretty much closes my request in issue #2703, so you might like to add that as a linked issue.

[jayhesselberth]

Thanks!