user-friendly documentation for plugin URL field

A user not knowing how plugins are used in Minerva will be left wondering what this is about:

At least a link to the documentation should be given. See also this documentation issue.

changed milestone to %v12.1.0

created branch 479-user-friendly-documentation-for-plugin-url-field

mentioned in merge request !379 (merged)

closed via merge request !379 (merged)

mentioned in commit 9bcf2651