Docs: Modify new resource handlers topic to apply to app plugins

[josmperez]

New resource handler topic could be modified to apply to app plugins. Suggested by Marcus in #824 (comment):

"This applies to both data source and app plugins with a backend so there's a nice opportunity to make it cover both. Up to you whether you want to do it in this PR or follow up later.

This example cover using resource handlers in an app: grafana/grafana-plugin-examples@main/examples/app-with-backend"