Roadmap board, docs sidebar and CMS changelog

Added

Introduces the public roadmap board, a persistent docs sidebar with scroll-spy, and a CMS-backed changelog so release notes stay in sync with what actually shipped.

Added

Public roadmap board at /roadmap, filterable by area and status.
Docs sidebar with automatic scroll-spy across every docs page.
CMS-backed changelog — this page — sourced from the Releases collection.

Fix stale cache headers on docs pages