Retire RAC-MIGRATION.md - the migration is complete

Transferable lessons moved to ../ui/docs/migration-playbook.md (gotchas
23-26, fidelity worked method and harness notes, roadmap/vocabulary
updates); the parked brand-ramp material to the private repo's
docs/brand-ramp-review.md; the pre-merge manual review checklist to
python-manual-review.md (untracked); drop historical content.

Author: microbit-matt-hillsdon