docs(community): restructure community docs - #3363

Summary

Restructures the Community section of the docs so each page is a prescriptive reference with a single home for each rule, instead of several pages restating the same process.

• Contributing is now a short entry point (where to start, docs/spec fixes, SDK work, protocol changes, where to talk, ground rules). The step-by-step git tutorial is gone; repo setup lives in CONTRIBUTING.md.
• Project Policies (new) collects the policies that apply to everyone: Code of Conduct, antitrust, licensing, AI-assisted contributions, and vulnerability reporting. It replaces community/security, which now redirects.
• SEP Guidelines, Contributor Ladder, Working Groups and Interest Groups, Communication, SDK Tiers, Feature Lifecycle and Charter Template are rewritten to cite their adopting SEP, drop duplicated content, and cross-link rather than restate (for example, escalation lives in the Contributor Ladder, meeting rules in Working Groups and Interest Groups).
• Working Groups and Interest Groups now lists the current groups, and the charter template has a "Submitting a charter" checklist.
• Nav: the groups are now Contributing, Roadmap, Governance (with WG/IG charters nested), and Policies. The Apps WG charter was missing from the nav and is now listed.
• CONTRIBUTING.md gains a short "Schema examples" section.

Open questions for reviewers

A few statements are new or tighter than before and should be confirmed:

• SEP review criteria now include fit with the roadmap and design principles.
• SDK Tiers says each official SDK runs conformance against its default branch and publishes results, while tier advancement is measured on the latest stable release.
• Project Policies says private vulnerability reporting is enabled on every project repository, not only the SDKs.
• Group meetings must now link Project Policies from their agenda.

Test plan

• prettier --check on the changed files
• mint broken-links: no broken links
• npm run check in CI