Document the pybabel translation workflow

[xlyy1]

The command reference describes individual options, but first-time users lack a complete example showing how catalogs are created and maintained. Add a Python gettext walkthrough covering extract, init, translation, compile, and update, plus Jinja extraction and runtime configuration. Link it from the command reference.

Fixes #1050.

Validation on Windows, CPython 3.13, Jinja 3.1.6:

• Ran the documented CLI workflow in a temporary project: fallback before compilation, German translation loading, updates preserving existing translations, and Python/Jinja extraction and rendering all passed.
• git diff --check passed.
• Sphinx 9.1.0 HTML builds with -W --keep-going report the same six existing warnings/errors on the base and this branch (Python inventory network failure, existing docstring indentation and unresolved targets); no new diagnostics.

The CLDR 48.2 download was unavailable locally, so the walkthrough and builds used generated locale data from the released Babel 2.18.0 package. No library code changed; the full runtime test suite was not run.

Prepared with assistance from OpenAI Codex.