docs: Add Phase 1.2.3 documentation standards to MODERNIZATION.md

Comprehensive documentation modernization standards including:

- Standardized project directory structure (src layout for all 6 repos)
- PEP compliance table (517, 518, 621, 660, 561, 639)
- Migration guide from flat layout to src layout
- Legacy files to remove (setup.py, setup.cfg, tox.ini, etc.)
- Dependency specification policy (libraries vs application)
- Production Dockerfile with uv sync --frozen --require-hashes
- Complete docs/conf.py template with:
  - Furo theme with Noto fonts
  - MyST Markdown configuration
  - sphinx-autoapi (no-import API docs)
  - Intersphinx cross-referencing for WAMP ecosystem
  - sphinx-design, sphinx-copybutton, OpenGraph
  - Spelling configuration
- Per-repo intersphinx mapping table
- Documentation dependencies in separate [docs] extra
- Standard .readthedocs.yaml template

Note: This work was completed with AI assistance (Claude Code).

Author: oberstet