DAST On-demand scans MVC - Initiate Scan - Documentation

Summary

This issue is for documenting the On-demand scans feature described in #218465 (closed). The documentation should be accessible at https://docs.gitlab.com/ee/user/application_security/dast/#on-demand-scans.

Implementation plan

• Add a new On-demand section in doc/user/application_security/dast/index.md
• Make sure that empty states' secondary button in the On-demand page properly points to the new section