[docs]: explain cloud-config sources and hooks

# Documentation request

Create a tutorial / how-to guide outlying from a high-level POV:

- how all the cloud-config sources interconnect
- what the default merge priority is / how to override the merge priority
- how different personas can hook into them
- how to inject instance metadata vars (jinja templating) in cloud-config
- pointing to the reference / explanation guides of these features

This idea comes from https://github.com/canonical/cloud-init/pull/4155 and other questions related to templating and these concepts on IRC.