release: bump 0.11.0 -> 0.12.0 — refresh vendored landing.html (v3)

Vendored landing page updated to the v3 shared asset (Markdown tables,
schema/function descriptions, two-tier Cupola CTA, INSTALL line, footer).
Conformance JSON Schema allows the new optional per-schema `doc` field.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: rustyconover