docs: Add BIND9 version comparison and migration guidance

- Generate BIND_VERSION_DIFFERENCES.md with detailed grammar comparison
- Document 44 breaking changes between BIND9 9.18.44 and 9.20.18
- Document 35 new options and 22 modified options in BIND9 9.20
- Document 3 newly deprecated options
- Add version compatibility section to CONFIGURATION_GRAMMAR.md
- Update CHANGELOG.md with version differences details
- Include migration guide for upgrading from 9.18 to 9.20

Closes #11

CHANGELOG.md

@@ -14,10 +14,16 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Grammar fetcher for upstream BIND9 sources (`scripts/fetch_bind_grammar.py`)
 - Automated version difference tracking
 - Operating system support matrix
+- BIND9 Version Differences documentation (`docs/BIND_VERSION_DIFFERENCES.md`) covering:
+  - 44 breaking changes between BIND9 9.18.44 and 9.20.18
+  - 35 new options in BIND9 9.20
+  - 22 modified options requiring configuration updates
+  - 3 newly deprecated options
 
 ### Changed
 - Established formal release management process
 - Defined backporting policy for security, bugs, and features
+- Clarified version compatibility in CONFIGURATION_GRAMMAR.md
 
 ### Planned
 - BIND9 9.20+ support in separate `9.20` branch