Add documentation about security policy bots

What does this MR do and why?

This adds a short section about security policy bot users to the schedule rule type documentation

How to set up and validate locally

1. Visit http://127.0.0.1:3000/help/user/application_security/policies/scan-execution-policies.md

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.