Adds initial documentation for Service Desk custom email

Feature context

Click to expand 👇

Right now it is not possible to customize the Service Desk email address (intake and sending) in its entirety. On self-hosted instances you have more control over the used addresses, but you will still have a rather cryptic target email address for a specific service desk in a project. For .com users it's currently not possible to customize the Service Desk email at all.

There is a proposal and a further exploration around this issue. A summary of the solution path is the following: Users set up their custom email to forward all emails to the cryptic Service Desk email and provide SMTP credentials so we can send emails on their behalf. This way customers seeking support will only see the custom email address in their communication.

There is further discussion about improving and changing the general infrastructure, but this approach is a MVC to solve the issue for our customers.

🗺 How does it contribute to the whole feature?

This MR is part of a series of MRs that will follow in order to complete this feature. See #329990 (comment 1227384943) for a detailed breakdown. Here's a summary:

1. ✅ Using SMTP credentials. Foundation work.
2. ✅ Verify email ownership, correct function and setup
3. ✅ Add settings and validation to Settings page.
   • Backend
     1. ✅ Create and Destroy Services
     2. ✅ CustomEmailController
   • Frontend
     1. ✅ CustomEmail component foundation with initial resource fetching
     2. ✅ CustomEmailForm component with resource saving
     3. ✅ CustomEmailStateStarted and CustomEmailConfirmModal components (with resource deletion)
     4. ✅ CustomEmailStateFailed component (with resource deletion)
     5. ✅ CustomEmailStateFinished component with resource update
4. Ingest replies from custom email
5. 🎯 Add documentation

What does this MR do and why?

🎏 This feature is hidden behind a feature flag and not used on production yet.

This MR adds a first and initial portion of documentation for the Service Desk custom email feature. The idea ist that administrators have a documentation document to go with the Beta release of the feature that will help them learn more about the feature itself, requirements, technical details and limitations.

Screenshots or screen recordings

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🚫 docs only change

How to set up and validate locally

🚫 docs only change

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