Improve Secret Detection MR docs

What does this MR do?

Provides detailed instructions on how to enable Secret Detection by manually editing the .gitlab-ci.yml file. Previously, the instructions only quoted the content to be added to the file.

Related issues

This MR is associated with the following issue:

• Improve error communication when enabling Secre... (#379206 - closed)

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doc/user/application_security/secret_detection/index.md

@@ -92,7 +92,7 @@ Use this method if you have an existing GitLab CI/CD configuration file.
 
 1. On the top bar, select **Main menu > Projects** and find your project.
 1. On the left sidebar, select **CI/CD > Editor**.
-1. Paste the following at the bottom of the `.gitlab-ci.yml` file:
+1. Copy and paste the following to the bottom of the `.gitlab-ci.yml` file:
 
    ```yaml
    include: