Add CI variables to provide GitLab port and protocol

What does this MR do?

• Adds two predefined CI environment variables CI_SERVER_PORT and CI_SERVER_PROTOCOL to allow the use of basic access authentication (e.g. https://gitlab-ci-token:${CI_JOB_TOKEN}@example.com:3000, to clone repositories or push changes) without hardcoding protocol or port.
• Adds documentation and examples of the new variables CI_SERVER_PORT and CI_SERVER_PROTOCOL.
• Closes: #197665 (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