Allow skipping documents (or paragraphs) in i18n

[jeanas]

Is your feature request related to a problem? Please describe.

On some projects, the amount of documentation is large, but only part of that documentation is nice to translate, while part of it should not be translated.

Two examples:

• Python documentation (https://docs.python.org) — The C API documentation is bulky, unlikely to be useful for people who don't understand English, and very technical. In the python-docs-fr repo (CC @JulienPalard), the README documents that one should not translate it.
• Python Packaging User Guide (https://packaging.python.org) — The tutorials are nice to have translated, but the PyPA specifications should not be.

Describe the solution you'd like

In such cases, it would be very helpful to be able to entirely skip generating catalog messages for a given reST/Markdown file or even directly. This would obviate the need to document what should/shouldn't be translated, and make it easier to have an accurate representation of the translation progress. (The Potodo tool has an option to exclude given PO files from the statistics, but that won't work if you're using a single POT/PO file for everything (gettext_compact = "mydoc"), and it doesn't help translators on platforms like Weblate.)

This should also be reflected in the |translation progress| substitution.

[jeanas]

One thing I'm not sure about is whether this should be configured via conf.py or via something in the reST/Markdown sources...

[n-peugnet]

I also thought one day that it would be useful if the only directive worked with the gettext builder.

Last time I checked, it is not possible to use the following to exclude some content from the translation:

.. only:: not gettext

   Text excluded from the translation.

Which feels strange given the description of the directive:

The format and the name of the current builder (html, latex or text) are always set as a tag.

At the same time I understand that the gettext builder should ignore the only directive most of the time, otherwise content inside .. only:: html for example would not be translated. But I think it could make sense to respect it if the gettext tag is explicitely excluded.