chore: setup richtext refactor infrastructure

- Add .doc/ folder for technical documentation (gitignored)
- Create technical design document for HTML→Markdown→swift-markdown migration
- Setup feature branch for richtext rendering refactor

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: graycreate

.gitignore

@@ -17,3 +17,6 @@ fastlane/.env*
 ## Match
 fastlane/certificates/
 fastlane/profiles/
+
+## Documentation (working notes)
+.doc/