Add a rustdoc user guide to https://doc.rust-lang.org/rustdoc

[jyn514]

Right now, https://doc.rust-lang.org/rustdoc is mostly developer-oriented, it has lots of info about how to use rustdoc the tool but not how to use the generated docs. We should add a user-focused page to it that mentions options people don't usually know about, e.g.

• ? for search tips
• the "In Parameters" and "In Return Types" tab of search
• the settings page maybe?

[NoraCodes]

Throwing my hat in the ring on this one. Let me know if you'd like me to add some docs for the settings page!

[jyn514]

This was added in #90487; thanks @NoraCodes !