Document explicit extraction mapping file selection - #1345

Summary

The mapping documentation describes INI and TOML syntax, but does not show how to select a configuration file. This leaves readers unsure whether a particular filename is required or whether pyproject.toml is loaded automatically.

• Show pybabel extract -F examples for babel.cfg, babel.toml, and pyproject.toml.
• Explain explicit selection, the default Python-only mapping, and filename-based TOML/INI handling.
• Correct the introduction to include the built-in JavaScript extractor.

The existing sections already describe built-in extractors, common options, and TOML syntax; this fills the remaining invocation gap. No runtime behavior changes.

Closes #1063.

Validation

On macOS with CPython 3.13.5, after importing the repository's CLDR 48.2 data:

• python -m pytest -q tests: 6502 passed, 1174 skipped, 1 xfailed. Skips include unavailable pytz, setuptools, and benchmark dependencies, plus platform/deprecation-specific cases. One existing pytest parametrization deprecation warning.
• python -m pytest -q tests/messages: 383 passed, 1 skipped.
• Extracted the three new CLI commands and corresponding INI/TOML examples directly from the documentation, then ran them against temporary Python and JavaScript files. Verified the resulting POT messages. Also verified that a pyproject.toml ignore mapping has no effect without -F and takes effect when explicitly selected. These were local behavioral checks, not new committed tests.
• git diff --check: passed.
• python -m sphinx -b html -W --keep-going docs <output>: not clean on either the unchanged base or this branch. Both report the same five existing diagnostics (JavaScript extractor docstring indentation and unresolved links in dates/dev/messages); no new diagnostic was introduced.

The full multi-version/platform CI matrix was not run locally.

AI assistance disclosure: this documentation change and local verification were prepared with Hermes Agent assistance.