doc: Reintroduce docs on running Sec templates using MR pipelines

What does this MR do and why?

This section was previously removed since it didn't provide a uniform method of supporting Merge Request Pipelines via Sec templates.

Since then we have finished merging support into all latest sectionsec templates per #217668 (closed)

Reintroduces section removed with !93969 (merged) in support of #217668 (closed)

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.