docs: update CHANGELOG with unreleased changes for v4.0.0 stable

Drafts the [Unreleased] section to cover everything queued up since
4.0.0-rc7: 18 new page templates, the Recommended Integrations docs
page, the in-topbar color-mode toggle, five issue fixes (#6010, #6019,
#6020, #6021, #6026, #6028), the forms-section restructure, the
Bootstrap 5.3.4 variable sync, the dependency-upgrade batch (ESLint 10,
TypeScript 6, Stylelint 17, Astro 6.3, etc.) and the legacy ESLint
config cleanup.