Chore(health): Add community health files

[JordyPirata]

Summary

Brings ambrosia-dev up to the same community/health-file bar as the main ambrosia repo, adapted for a Docusaurus docs portal, plus a few leftover scaffold fixes found along the way:

• Add CONTRIBUTING.md, CODE_OF_CONDUCT.md, and SECURITY.md, adapted for this repo (docs site, not the client/server/electron app).
• Add .github/ISSUE_TEMPLATE/ (bug report + feature request, adapted for docs — affected page/locale instead of OS/smartphone fields) and a dependency_review.yml workflow, mirroring the main repo's supply-chain checks.
• Fix two docusaurus.config.js editUrl values left over from scaffolding: docs.editUrl pointed at the wrong repo (ambrosia instead of ambrosia-dev), and blog.editUrl was still the unedited facebook/docusaurus template placeholder.
• Trim .gitignore: it was a verbatim copy of facebook/docusaurus's own monorepo .gitignore (packages/, website/, Crowdin, Netlify, rspack-tracing, yarn — none of which apply here, since this project uses npm and has no website//packages/ subdirectories).
• Add license/repository fields to package.json.
• Move find_unused_translations.sh into scripts/, alongside install.sh/uninstall.sh.
• Polish README.md: license badge, project description, link to the live site, and a Contributing section.

Test plan

• npm run build succeeds after the docusaurus.config.js/package.json changes.
• git status clean after each commit — no node_modules, build, or .docusaurus leaking in under the trimmed .gitignore.
• bash scripts/find_unused_translations.sh still runs correctly from its new location.
• Internal links between the new CONTRIBUTING.md / CODE_OF_CONDUCT.md / SECURITY.md / issue templates resolve correctly.

[Sharmaz]

LGTM!!

[ca-ruz]

LGTM!