Improve documentation about how to override Secure templates with custom config

The following discussions from !93424 (closed) should be addressed:

• @sethgitlab started a discussion: (+9 comments)

  save on usage of CI minutes or shorten your pipeline run time, prevent the DAST job running on MRs where the title starts with the

• @rdickenson started a discussion: (+7 comments)

  @marcel.amirault - Could you please review this MR?

  The tip documented in this MR can be applied to all pipelines. However, I wanted to provide a DAST-specific example. After this MR is merged, I suggest we link to this as an example from https://docs.gitlab.com/ee/ci/pipelines/cicd_minutes.html#reduce-consumption-of-cicd-minutes. I first thought of putting the content of this MR in that section, and linking to it from the DAST docs. However, it would seem odd to include a DAST-specific example in general CI/CD docs. I'm open to other ideas.

Proposal

While reviewing !93424 (comment 1050229938), we realized we probably need a new page to deal with advanced configuration details for CI/CD templates. See linked discussions for more details.

This issue is to figure out what we should document and where it should be.