docs: make configuration defaults authoritative - #3000

Summary

• point documentation at configuration.toml as the authoritative source for mutable defaults
• replace line-number deep links with stable file links
• remove duplicated default values from the review, describe, and improve option tables
• decouple the MOSAICO health-test fixture from the shipped default model

Closes #2960.

Validation

• PYTHONPATH=. .venv/Scripts/python.exe -m pytest tests/unittest/test_mosaico_health.py -q — 8 passed
• mkdocs build -f docs/mkdocs.yml — passed (repository-existing link/imaging warnings remain)
• Ruff check plus the v6 pre-commit hook entry points for AST, conflicts, case conflicts, symlinks, file endings, line endings, trailing whitespace, and debug statements — passed
• git diff --check — passed

The aggregate pre-commit run --files ... command could not initialize its remote hook environment because GitHub returned a port 443 timeout; the same pinned hook version was installed from PyPI and run directly instead.