Add documentation link in secrets push check

What does this MR do and why?

This merge request adds a link to the documentation page for pre-receive secret detection in the secrets push check.

Resolves #435093 (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.

Screenshots or screen recordings

Below is a before and after for the push check with the documentation link added.

Before	After

How to set up and validate locally

• Create a new project or use an existing one.
• In your rails console, execute the following commands:

pry(main)> Gitlab::CurrentSettings.update!(pre_receive_secret_detection_enabled: true) # to ensure feature is enabled instance-wide

pry(main)> project = Project.find(PROJECT_ID)
pry(main)> Feature.enable(:pre_receive_secret_detection_push_check, project) 

• Ensure your GDK is licensed as ultimate.
• In your terminal, navigate to the project folder.
• Create a new file, e.g. .env, and add a gitlab personal access token:

TOKEN=glpat-JUST20LETTERSANDNUMB

• Run git add . and git commit -m 'test' to commit the file.
• Run the command to push the commited file git push.
• Verify the push fails due to the secret detected and it shows the documentation text/link.