Update code quality documentation on how to use with MR pipelines

What does this MR do?

This is an outcome of the discussion from !33262 (closed).

Currently, the vendored template for Code Quality is the same one used by Auto DevOps, and is not meant to run the code_quality job in MR pipelines out of the box. This MR updates the documentation to include an example on how to make it work with MR pipelines while still using the vendored template.

This documentation update will also help on guiding how to update the configuration on #218797. This is because a lot of our projects enable pipelines for merge requests.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team