inti_site() should be able to rewrite dependencies in all pages #3025

I'm iterating on a pkgdown site via _brand.yml and frequently running into problems when a site change affects things written into docs/deps/data-deps.txt (typically changing fonts is the easiest way to trigger this).

Changes in these dependencies then require that every page be rebuilt to pick up the change in data-deps.txt, since it holds <script> and <link> tags that need to be injected into the header of every page.

It'd be very helpful if pkgdown::init_site() could handle that kind of change automatically. My recommendation (that I haven't tested and that may not work) is to use comment markers around the dependencies managed by pkgdown so that init_site() can do a site-wide find-and-replace.

Sidenote: I this kind of change could also be useful for any navbar updates that need to be propagated everywhere in the site.