Update and clean up Plugin infos

[MyPyDavid]

• references to rdmo-plugins should be removed
  • https://rdmo.readthedocs.io/en/latest/development/setup.html#setup-plugins
• Only general infos about plugins should be kept and the specific parts should be removed, each specific plugin should explain in their own READMEs how they can be installed and configured.
  • https://rdmo.readthedocs.io/en/latest/plugins/index.html

PS
For RDMO 2.6 (or 3.0 or after rdmorganiser/rdmo#1436) I should also add the infos about the new "Plugin" element, that can be configured via the Management UI