Remove image references to user-images

[evegufy]

Description

There are quite some references to user-images in the documentation:
https://github.com/search?q=repo%3Aeclipse-tractusx%2Fportal-assets%20user-images&type=code

This needs to be clean up.

All Images should be moved into the docs/static directory and referenced from there within the documentation.
The links need to be absolute as the docs app isn't enabled for relative paths yet.

Please also enhance the 'How to contribute' guide with a description about how to manage images (upload to static/docs, create license file, link in the documentation...).

Follow up task to #239