doc: Add source .po files for translations

This helps the Weblate translation workflow and offline translations
through .po files, it's important to update these files constantly.

The goal here is to have a semi-automated translation workflow; Weblate
will pull updates automatically from the main repository and commit
translations to another repository.

Doc committers are encouraged to update the source .po files whenever
there are updates in the English documentation part.

$ ./tools/update_translate_template.sh documentation

Anyway, I'll keep doing this periodically.

Discussed with: doceng