Add more details on how to use Docker-based Gitlab with Docker-based PlantUML

What does this MR do?

This MR adds information about setting up Docker-based PlantUML with Docker-based Omnibus deployment.

The path to the PlantUML service in the original version is misleading, as the Docker-based PlantUML is accessible at http://server/ and not http://server/plantuml/.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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