Docs: Consider enhancements to "nested plugins"

This page is lacking detail and probably needs revisiting to ensure we are providing appropriate guidance.

Questions / considerations:
- The opening statement claims app plugins can nest datasources and panels to provide a complete user experience
- Does this make Apps appear as simply a bundling mechanism? Do we want to recommend apps as a package?
- Does it conflict with general guidance on creating apps with custom **pages** to provide this "complete user experience"
- The instructions are minimal, they just say scaffold an app and then show a folder structure.
- Whether the idea is to bundle existing DS/Panels or create new ones, we should provide more complete instructions on how to do so.
- Setting a dependency on a version for a nested plugin can be... complicated, we should ensure that developers are aware of this

Originally posted by @sympatheticmoose in https://github.com/grafana/plugin-tools/pull/333#discussion_r1296187547