Adds overview video to Service Desk custom email doc

What does this MR do and why?

We have a video that showcases the custom email for Service Desk feature. This was originally recorded for the release post item, but we should also add it to the documentation as it walks through the whole setup process and gives an overview of the functionality.

Screenshots or screen recordings

🚫 docs only

How to set up and validate locally

🚫 docs only

MR acceptance checklist

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