Remove mergedeep dependency

[shenxianpeng]

Summary

Remove the unmaintained mergedeep runtime dependency by replacing its only use with a small internal deep-merge helper for inherited MkDocs configuration.

fixes mkdocs/mkdocs#4050

Details

mergedeep was only used when loading YAML configs with INHERIT. The new helper preserves the existing behavior needed by MkDocs:

• nested mappings are merged recursively
• child values override parent values
• lists and scalar values are replaced rather than concatenated

The dependency is also removed from the main dependency list and the min-versions optional dependency set.

Validation

• ./.venv/bin/python -m unittest mkdocs.tests.utils.utils_tests
• ./.venv/bin/python -m unittest discover -s mkdocs -p "*tests.py"
• git diff --check
• commit hook checks: ruff check, ruff format, isort, codespell, TOML validation