Create a plugin guide for publishing to custom remote repository

Currently there's a guide that shows how a plugin may be published to the Gradle Plugin Portal but there's no indication on how to do the same with a custom remote repository (such as Nexus, Artifactory, etc). The `java-gradle-plugin` generates a plugin marker publication that should also be pushed to the remote repository but it's not configured to do so by default. this is one of the spets that should be covered in this new guide.

Read the existing guide for publishing plugins to the Gradle Plugin Portal and review the java-gradle-plugin behavior around generated plugin marker publications. Cover publishing a plugin and its marker publication to a custom repository such as Nexus or Artifactory. Done means the new guide explains the required configuration for the complete publishing process.

Written by the indexing model from the issue text.