doc: add minimal smart rollup monitoring info

What

List metrics available for rollup node.

Why

They exists, and we have a dashboard coming, so we should document them.

How

The monitoring is very close to the one for the octez node, so we just point to it, and give a list of metrics, obtained using the dump-metrics command.

Manually testing the MR

CI ?

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR