Draft: Common contact footer

• UJ: Implement standard first version of a template, using a mixture of Example 3 and 4 from #296 (closed), for the time being.
  • Standard footer now referenceable via {% include contact_info.md %}
  • 👉 template location
• UJ: Replace for some pages ...
• Replace for all pages (more or less)
  • Special solution for Spotlights needed (but not obligatory for this MR)
• LK: Decide on final template such that the content & format fits to all pages
  • Additional link to newsletters abo?
  • Additional link to "How-to-contribute"?

Closes #296 (closed)