docs: Split reference documentation into a self-standing guide

[gdemonet]

Component: docs

Why this is needed:

Today, reference information (such as TCP ports requirements, list of Prometheus recording rules, ...) is scattered across the Installation and Operation guides.

What should be done:

Extract these details into a Reference guide, and link to it from other guides when relevant.

[gdemonet]

Additional note (see https://github.com/scality/metalk8s/pull/3074/files#r583908179): verify that tables (since we'll have many of those in such a guide) are properly rendered for PDF output.