changelog: split into firmware/IDE products and add full OpenMV IDE changelog

Split the changelog landing page into two products and move the existing
firmware pages under changelog/firmware/. Add a complete OpenMV IDE changelog
under changelog/ide/: per-major index pages plus 70 release pages
(v1.0.0 - v4.8.11), reconstructed from the actual qt-creator commit diffs for
each openmv-ide release tag (Qt Creator base 4.0.2 -> 10.0.1 -> 14.0.2). Every
entry links its commits to github.com/openmv/qt-creator.

- changelog/index.rst: product hub (Firmware | IDE)
- changelog/firmware/: existing pages moved here (label _changelog_firmware)
- changelog/ide/: new IDE changelog (index, v1-v4 majors, 70 releases)
- static/custom.css: .changelog-products styling
- templates/topindex.html: point the "What's New" strip at changelog/firmware/

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

Author: kwagyeman