Add docs for using a custom scanning stage

What does this MR do and why?

This MR adds a new Using a custom scanning stage section to the Application Security docs.

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.

Related to #218541 (closed)