Improve automation and documentation related to translations

[spier]

Follow-up work from #613:

• ✅ Regenerate the mindmap for Galician automatically. We will have to re-work the generate-mindmap.yml GitHub Action, to make sure that we re-generate the mindmap for Galician as well, if the input files change.
• [maintainer docs] describe how maintainers need to update the GitHub Action workflows when a new language is added. see 241bdf2
• [maintainer docs] add new translation lead (ask; make code changes as shown in 95b8fc7)
• [docs] show the possibility to translate the images as well (Galician was the first language where this was done 🥳 )

Example showing the folder structure with all asses for a new language

The screenshot is from file view of PR #613. Maybe we could annotate the screenshot to explain which folders need to be populate, and which ones are optional?