Docs: Document app (plugin) and datasource proxy internals

[marefr]

The app (plugin) and datasource proxy internally works similarly, but still are different. Sometimes both are referred to as plugin proxy which is unfortunate and confusing. Would be good to document the internals as a first step. We do have http://localhost:3000/developers/plugin-tools/how-to-guides/data-source-plugins/add-authentication-for-data-source-plugins, http://localhost:3000/developers/plugin-tools/how-to-guides/app-plugins/add-authentication-for-app-plugins and http://localhost:3000/developers/plugin-tools/how-to-guides/data-source-plugins/fetch-data-from-frontend that might need some changes/additions to make this topic more easily understandable.

Maybe after this it's easier to see a way forward to refactor into one proxying implementation that works the same for both apps and plugins.