docs: provide guidance for use of plugin-validator for detecting Angular in private plugins

[sympatheticmoose]

To help OSS/Enterprise customers who have developed custom private plugins discover those which are based on Angular.

The hope is this is a quick win as we build out more visibility within Grafana itself. But if we/CX can link people to explicit steps with a reference anchor that will be a good first step.

Suggestion would be a new section in the readme #Detecting if a private plugin is based on angular

We could also consider updating https://grafana.github.io/plugin-tools/ but afaik validator is not listed there (yet).

[sympatheticmoose]

Other solutions outlined to better support this use case