Guide for plugin maintainers: How to update plugins to Maven 4

### Improvement proposal

On Javaland 2025 conference the question about how to update plugins to Maven 4 as a plugin maintainer / where to find an guide for that raised multiple times (in the Maven panel, during the talk of Karl Heinz and me, in personal chats) and we had to admit that there is none (yet).
We need to change that.

Edit:
Topics from comments we should mention:

* maven 4 will support well maintained Maven 3 plugins
* plugin maintainer will have choice to create a Maven 4 specific release of their plugin, that won't work in Maven 3
** They need to decide if and how long they want to do this.

Start by reviewing the Maven site's existing plugin documentation and the issue's listed Maven 4 topics. The guide should explain Maven 3 plugin compatibility, the option of a Maven 4-specific release, and the maintainer's version-support decision; the issue names no files or tests.

Written by the indexing model from the issue text.