Adds docs to generic_alert.md

What does this MR do?

• Updates documentation to set up generic alert endpoint

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content

https://gitlab.com/gitlab-org/gitlab-ee/issues/13203

This is the MR for updating the link: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/16484/diffs