Add description and example uasge for `Plugin.FromSources` to `reference.conf` file

[LukaszKontowski]

org.virtuslab.ideprobe.dependencies.Plugin.FromSources could be used in probe.intellij.plugins array. It looks like FromSources usage is dedicated to users, who interact with ide-probe rather by direct code calls (and not by the .conf file), because it requires the user to have a custom implementation of DependencyBuilder and register it by using org.virtuslab.ideprobe.dependencies.DependencyProvider#registerBuilder

For now, I did not manage to find it's real-life usage in ide-probe nor in known repositories, which use ide-probe.

[PawelLipski]

TBD if it needs to be maintained at all!

[lukaszwawrzyk]

It is useful in config. It requires adding code, yes, but it doesn't mean it should not be in the config.
There are existing builders for scala and pants plugin. It requires to add PantsPluginExtension in your code and then in the config you can put this as plugin:

{
      id = "pants",
      repository = { path = "https://github.com/tpasternak/intellij-pants-plugin.git", ref = "bump-203-pr-1.26.x" }
      env = { "PANTS_SHA" = "1.25.x-twtr" }
    }

So then it is easy to test your code agains prereleased plugin versions from custom branch/commit from fork or official repository.