Document the contextual keyword `raw`

[CrazyboyQCD]

Related with #34601, #127679.

[fmease]

While it would be straightforward to add a "keyword page" for raw by adding the following to library/std/src/keyword_docs.rs:

#[doc(keyword = "raw")]
/// ...
mod raw_keyword {}

I think ideally we want to mark it as a contextual keyword in some way and e.g., reflect that characteristic in the section header prefix.

[fmease]

However, we don't mark union specially either despite it being a contextual keyword, too. So I guess, take it away!

[fmease]

Only tangentially related: We should probably also document the Rust ≥2024 contextual keyword safe I just noticed.