Add a footer to the doc pages (with trademark notes)

[razvan]

Add a footer to the docs pages. This could contain links to the GH repos, contributor's guide and so on.

Here is an example from the Kubernetes docs:

Edit (Felix): The footer should also contain the trademark notes, like on our main page:

Acceptance criteria

• all the docs pages have the same footer as the main page

Steps

• copy-paste the footer into this repo
• update the UI repo reference in the documentation and rebuild

[fhennig]

This needs to be done in the UI, I'm moving the ticket there