Auto merge of #86157 - jsha:tera, r=jyn514,GuillaumeGomez

Auto merge of #86157 - jsha:tera, r=jyn514,GuillaumeGomez

Use Tera templates for rustdoc.

Replaces a format!() call in layout::render with a template
expansion. Introduces a `templates` field in SharedContext so parts
of rustdoc can share pre-rendered templates.

This currently builds in a copy of the single template available, like
with static files. However, future work can make this live-loadable with
a perma-unstable flag, to make rustdoc developers' work easier.

Part of #84419.

Demo at https://hoffman-andrews.com/rust/tera/std/string/struct.String.html.