Document localisation and internationalisation workflow

[awvwgk]

Generation of new languages is currently automated using make gettext to update or generate new translations:

https://github.com/awvwgk/fpm-docs/blob/03f2691ddd69c7c4d42b770018eafeec9ef761ea/Makefile#L26-L28

The sphinx-build and sphinx-intl commands take care of most steps from the usual translation workflow, the translator only has to fill in the translations in the po files and check them into the repository.

Relevant links:

• settings for pages/conf.py:
  https://www.sphinx-doc.org/en/master/usage/configuration.html#options-for-internationalization
• sphinx-intl:
  https://www.sphinx-doc.org/en/master/usage/advanced/intl.html#intl
• gettext, msgfmt, po-format:
  https://en.wikipedia.org/wiki/Gettext

Once we smoothed out the workflow for translating the documentation, a short recipe with the most important points should be available in the How-To section (could also profit from potential translations).