Fix a link to docs for Mask CI/CD variable from Settings

What does this MR do?

Fixes a link to docs for Mask CI/CD variable from Settings since the current anchor is dead due to !57464 (merged).

Screenshots (strongly suggested)

https://gitlab.com/<project>/-/settings/ci_cd -> "Variables" section -> "Add Variable" button:

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have not included a changelog entry because docs-linting.
• Code review guidelines
• Style guides
• Separation of EE specific content