Doc updates for Project Level Alert Dashboard MVC

Problem to solve

Documentation needs to be available when we launch the Project Level Alert Dashboard MVC.

Further details

Typically documentation is included with the feature change. Since we've broken this feature in an iterative fashion & hid all functionality behind a feature flag, the documentation has not been delivered with each implementation issue.

Proposal

Documentation will be generated (after the fact) based on the final MVC requirements under the scope of this issue. This will require frontend & backend contributions as well as assistance from our TW counterpart @ngaskill.

Who can address the issue

Other links/references

Verify that documentation was either done or is not needed for these MRs/Issues:

• backend !50586 (merged). Possible update: document app.gitlab.com/alert: "true" annotation in https://docs.gitlab.com/ee/user/application_security/threat_monitoring/
• backend !48515 (merged). Possible update: add /internal/kubernetes/network_alert to https://docs.gitlab.com/ee/development/internal_api.html#kubernetes-agent-endpoints
• frontend docs: !53776 (merged)