Document Duo Chat E2E tests
What does this MR do and why?
Adds development documentation for GitLab Duo Chat end to end tests.
As suggested in https://gitlab.slack.com/archives/C06D5C70MD2/p1721028831980149
MR acceptance checklist
Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.
Edited by Jay McCure