[smaranjit]

What does this change?

Helps people find the project when they search for the problem it solves, such as "get a Firebase ID token for Postman" or "test an API that requires Firebase auth", rather than for the tool's name.

• New FAQ page (docs/src/faq.md, linked from the guide's sidebar and front page). Each heading is a question people search for, answered briefly with a link to the full page:
  • getting an ID token for Postman or curl, and testing a backend that runs verifyIdToken;
  • signing in as a user without their password;
  • setting custom claims without code, and why new claims don't appear in a token;
  • App Check tokens for testing, and restricted Android and iOS keys;
  • decoding JWTs, how long tokens last, the Auth emulator, supported platforms, and whether it's free.
• Docs site description (docs/book.toml): rewritten in those terms. It's the <meta name="description"> on every page of the site.
• README: a second intro paragraph about using the token in Postman, curl or your own tests, with a link to the FAQ.
• Guide front page: the same use case in the first paragraph, plus a pointer to the FAQ.
• Cargo.toml keywords: firebase, firebase-auth, id-token, jwt, app-check, replacing auth, gui, egui. These only matter if the crate is published.

Already applied outside this PR with gh repo edit:

• A new repository description, phrased the same way.
• 20 topics, including firebase-auth, firebase-token, id-token, custom-claims, firebase-app-check, jwt-decoder, api-testing and backend-testing.

Checklist

• cargo fmt --all -- --check passes
• cargo clippy --all-targets -- -D warnings passes (metadata-only Rust change)
• cargo test --all-targets passes
• cargo build --release succeeds
• Tested manually against a Firebase project (a development one): not applicable, docs and metadata only
• CHANGELOG.md updated under [Unreleased], if user-facing: not user-facing in the app

Notes for the reviewer

• Accuracy: every FAQ answer was checked against the app's behaviour. It says plainly that the app doesn't work with the Auth emulator and doesn't verify signatures, rather than implying otherwise.
• Checks: mdbook build docs builds with no warnings. All 20 pages' internal links and anchors resolve.
• Two things only the repo owner can do:
  • Upload a social preview image under Settings → General.
  • Register the docs site in Google Search Console.