[i18n] Add initial stubs for translating the docs

[sneakers-the-rat]

This PR adds some initial .po files for adding translation to the python docs.

It contains

• A python script to generate/update all configured language files
• A CI action that checks that any changes to the source english documents are reflected in the .po files
• Some initial threadbare translation docs
• The generated .po files for a few languages that we had volunteers to translate.

Questions for the JOSS:

• Do we want to just enable all languages that have some partial translation (anything is better than nothing) with a note that we welcome contributions to complete the translation? or should we only publish translated docs that are above some threshold of completeness?
• Readthedocs handles building multiple language versions ( https://docs.readthedocs.io/en/stable/localization.html#projects-with-multiple-translations ), but we need the project maintainers to add each language as a subproject before they appear. Does that need additional documentation or is that self-explanatory?
• What does review look like? do we need to do code-review style multiple readers? Or what!? ROpenSci has some pretty good docs on this that we're learning from over in pyopensci :)