docs: document 3.0→4.0 breaking changes and migration guide

[coretl]

Target page

reference/changes

Category: verify-then-write

The existing source (github.io reference/migration_guide.rst) only covers
2.0→3.0. The 3.0→4.0 section is completely absent.

What a human must do (~3 hours)

1. Review the PandABlocks 4.0 release notes, CHANGELOG, and git log for
   breaking changes since 3.0.
2. Check the existing migration_guide.rst (2.0→3.0) for style reference.
3. Write a 3.0→4.0 section for reference/changes covering:
   • Config file changes (boot.txt → config.txt, etc.)
   • Wire-protocol breaking changes (if any)
   • Removed / renamed blocks or fields
   • Build toolchain changes
4. Also add a compatibility matrix row for the 3.0→4.0 transition alongside
   the existing release_compatibility.rst content.

Post the draft as a comment or PR against the docs branch.

Source

• docs/_legacy_rst/ (github.io copies) — reference/migration_guide.rst
  (119 words, 2.0→3.0 only)
• reference/release_compatibility.rst (32 words)