docs: include Python library keywords in generated libdoc output

[KingParmenides]

Summary

• collect Python keyword libraries from lib/**/*.py while building the combined libdoc resource
• read @keyword decorators with Python AST so docs generation does not need to import Python libraries or satisfy constructor arguments
• emit keyword documentation and arguments into the generated resource alongside existing .robot keywords

Fixes #602

Verification

• python3 scripts/create-docs.py /private/tmp/all-keywords-with-py.robot
• /private/tmp/osfv-docs-venv/bin/libdoc /private/tmp/all-keywords-with-py.robot /private/tmp/all-keywords-with-py.html
• confirmed generated resource contains menus.Check if menu line is an option, QemuMonitor.Add HDD To Qemu, and fan_curve_tests.Plot Fan Curve
• /private/tmp/osfv-docs-venv/bin/black --check scripts/create-docs.py
• /private/tmp/osfv-docs-venv/bin/isort --check-only scripts/create-docs.py
• PYTHONPYCACHEPREFIX=/private/tmp/python-cache-osfv python3 -m py_compile scripts/create-docs.py
• git diff --check