doc: Missing documentation on configuration

[Huy-Ngo]

Overview Description

The page https://babel.pocoo.org/en/latest/messages.html has description of configuration format, however it does not mention the configuration file name anywhere, nor does any other pages.

The current description is also vague on which configuration options and which extractors are supported by default.

Expected Results

The documentation has details of info above.

[akx]

There is no single configuration filename; you'd use

  -F MAPPING_FILE, --mapping-file=MAPPING_FILE, --mapping=MAPPING_FILE
                        path to the mapping configuration file

to specify one when running a Babel CLI command. This is (admittedly vaguely) documented over at https://babel.pocoo.org/en/latest/setup.html#method-mapping (which is linked to from the page you mention).

Improvement PRs are welcome!

[tomasr8]

Just putting it here, we should also add documentation for the new TOML config: #1108