Add Sidekiq service desk and mail room instructions

Update 2023-01-30

Webhooks method now works Improves Service Desk Setup instructions for We... (!2955 - merged)

MR closed in favour of that method.

What does this merge request do and why?

Webhooks method of using service desk locally doesn't work without some authorisation hacks. (@msaleiko could you link an issue describing this problem if/when you get a chance?)

This MR adds instructions for the non-recommended Sidekiq approach. Doing so unblocks reviews of Service Desk MRs in the meantime.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Docs only change

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.