Add JUnit screenshots documentation

Part of #202114 (closed)

What does this MR do?

This MR adds the documentation associated to our new feature Displaying JUnit screenshots in the pipeline details page.

It adds some notes how this feature will work after completion #6061.

I tried to follow our guidelines, this is the first time I'm documenting a feature so please let me know what we should improve.

Screenshots

You can see how this feature will look like once #6061 will be finished.

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