[smaranjit]

What does this change?

Adds a user guide as an mdBook under docs/, published to GitHub Pages at https://smaranjit.github.io/firebase-token-toolkit/.

• Pages:
  • Getting started: installation, preparing a Firebase project, a quick start.
  • Using the app: the top bar, profiles, the Users panel, one page per tab, reading results.
  • Reference: settings storage, the Google endpoints called, troubleshooting with each error message and its fix, security, building from source.
• Screenshots: every screenshot and the README's quick-start GIF were captured from the v0.1.6 Linux build against a demo Firebase project.
• Workflow: new .github/workflows/docs.yml builds the book with a pinned mdBook 0.5.4 on PRs that touch docs/, and deploys to Pages from main.
• Fixed docs that no longer matched the UI:
  • The persistence checkbox is labelled remember API key & App ID, not "Remember secrets".
  • That checkbox also gates the App Check debug token.
  • The tab names now match the app.
  • SECURITY.md no longer describes a footer GitHub link.
• README and README.txt link to the guide. CHANGELOG and CONTRIBUTING are updated.

Checklist

• cargo fmt --all -- --check passes (no Rust changes)
• cargo clippy --all-targets -- -D warnings passes (no Rust changes)
• cargo test --all-targets passes (no Rust changes)
• cargo build --release succeeds (no Rust changes)
• Tested manually against a Firebase project (a development one)
• CHANGELOG.md updated under [Unreleased], if user-facing

Notes for the reviewer

• Docs checks:
  • mdbook build docs builds with no warnings.
  • Every internal link and anchor resolves, and all images load with alt text, checked in a browser at desktop and phone widths.
  • Every UI label quoted in the guide was checked against src/ui.
  • actionlint passes on the new workflow.
• Pages setup: the Pages source is set to GitHub Actions. The site deploys on the first push to main that touches docs/.
• Stacked PR: Support web, Android and iOS apps per profile #20 builds on this branch and updates the guide for multiple apps.