Add Pre-receive Secret Detection Documentation

What does this MR do and why?

Resolves #431766 (closed)

Add Pre-receive Secret Detection documentation.

Since Pre-receive Secret Detection is a new feature that will get triggered on every git push to an instance (when enabled), we need to provide clear accompanying documentation in the form of a blog post to assist our customers if there are questions about how the feature works.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

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.