Add dev docs on testing workspaces integration with KAS

Issue: Document steps required to test workspaces chan... (#560764 - closed)

What does this MR do and why?

Adds developer-facing docs to test WIP workspaces-KAS integration as part of the removal of workspaces proxy.

References

• Document steps required to test workspaces chan... (#560764 - closed)
• Simplify Workspaces setup by removing GitLab Wo... (gitlab-org#16785)

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Before	After

How to set up and validate locally

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