Add CI variable to provide GitLab base URL

What does this MR do?

• Adds a predefined CI environment variable CI_SERVER_URL that provides the full base URL of the GitLab instance (like https://gitlab.example.com:3000). As opposed to existing variable CI_SERVER_HOST which contains neither protocol nor non-default port.
• Adds documentation and examples of the new variable CI_SERVER_URL.
• Closes: #19751 (closed)

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