doc: add details on setting up Prometheus

What

Add more details on the instructions to install and set up Prometheus for monitoring the node.

Why

Some people complained that the existing instructions at <https://tezos.gitlab.io/user/node-monitoring.html#prometheus> were not sufficient.

How

Copy and adapt more detailed instructions from an internal document for installing the micro-machines.

Manually testing the MR

Read the additions and check if they are correct.

Checklist

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