Adds automation to build catalog templates.

[pierreozoux]

This is the necessary first step to then automate transifex fetch/push with this yml:

filters:
- filter_type: dir
  file_format: PO
  source_language: en
  source_file_dir: user_manual/locale/source
  # path expression to translation files, must contain <lang> placeholder
  translation_files_expression: 'user_manual/locale/<lang>/LC_MESSAGES'

Once the PR is merged, the github action should populate the folder user_manual/locale/source with a new commit.
Then we can configure transifex to fetch and push.

Translated strings should appear in user_manual/locale/<lang>/LC_MESSAGES

This PR superseeds #1824 I'll still need to make another PR to automatically build all the translated docs, but I need the files locally, hence this first PR :)

[skjnldsv]

Looks good, see comments :)
We'll have to merge to test it 🙈

[skjnldsv]

Translated strings should appear in user_manual/locale/<lang>/LC_MESSAGES

Same as #2223 (comment)
Can we use l10n instead? For consistency

[pierreozoux]

Yes, no problem for the last comment.

[pierreozoux]

Ok, I think we are good :)