Add multi node environment docs for Service Desk

What does this MR do and why?

Right now our documentation does not state how to set up Incoming email and Service Desk email ingestion properly on multi-node environments. This MR tries to close that gap adding a first iteration on multi-node email ingestion considerations.

Solves Improvements for Incoming Email feature (Mailro... (#391859 - closed)

Screenshots or screen recordings

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How to set up and validate locally

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MR acceptance checklist

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