Follow up to [#50527][]

Highlight the presence of the `yield :head` pattern established by
`turbo-rails` and adopted by Rails as part of [#50527][].

New applications will generate their application layout with `yield
:head`, so mention that in the documentation.

Also include the `yield :head` call in generated plugin layouts.

[#50527]: https://github.com/rails/rails/pull/50527