[Good First Issue]: Add Header<T> example to documentation and rustdoc

[juv]

Summary

The current documentation only outlines the Headers extractor: https://userapina.com/docs/core-concepts/extractors/#headers

Rapina includes another extractor, Header<T>, which gives access to a single header and is preferred to be used if only individual headers are required as per comment in the Headers extractor impl:

Prefer Header<T> when you know the header name upfront.

The Header<T> extractor and the remark to prefer Header<T> when accessing a single header with a known name is not reflected in the documentation.

Why This Matters

Clear examples in the documentation and a guideline when to prefer which of the two similar extractors.

Relevant Files

Extractors docs: https://github.com/rapina-rs/rapina/blob/main/docs/content/docs/core-concepts/extractors.md
Header<T> impl: https://github.com/rapina-rs/rapina/blob/main/rapina/src/extract/header.rs#L33-L36
Headers impl: https://github.com/rapina-rs/rapina/blob/main/rapina/src/extract/mod.rs#L140-L166

The Header<T> was added in pull request #542

Acceptance Criteria

• Documentation page for Rapina Extractors is updated
• Header<T> Rustdoc is updated with a clear example

Estimated Effort

XS — < 30 minutes

Hints & Resources

• Follow existing patterns in the documentation / rustdoc

Maintainer Checklist

• Issue is self-contained and scoped for a first-time contributor
• Relevant files and context are provided above
• Acceptance criteria are clear and verifiable

[popsiclelmlm]

/take

[ShiraiEd]

apparently @popsiclelmlm deleted the pr's origin repo,
So this became unassigned.

Now anyone can take this on