feat(docs): update site for P3 rcs

[ericgregory]

Updates the site ahead of the P3 release:

• New "Releases" section replaces the old flat "Interfaces" page, with a landing page (summary table, proposals, versioning policy) and three subpages
• New WASI P3 subpage covering native async motivation (sandwich problem), new primitives (async func, stream, future), common patterns, P3 worlds, per-interface change summaries, runtime and tooling support, and a P2-to-P3 migration section
• New WASI P2 subpage covering worlds, wasi:io, Component Model relationship, interface table with descriptions, runtime support, and patch release history
• New WASI P1 subpage covering POSIX-inspired design, API surface by category, limitations vs. P2+, and runtime support
• New Security page covering capability-based security, the contrast with ambient-authority models, and how capability grants evolved across P1/P2/P3
• New WASI test suite section on the Releases page, with a corresponding entry on the Resources page
• Description column added to the active-proposals tables, and the Phase 1/2 tables sync'd to current upstream
• Archived wasi-* subrepo URLs rewritten to canonical monorepo paths
• Updated introduction, roadmap, and resources pages to reference P3, surface the capability-based sandbox model, and link the Component Model book for tutorials