Add install GitLab runner popup

What does this MR do?

This adds a modal to the CI/CD settings page that shows up the instructions on how to add runners depending on the architecture and platform desired.

The changes are behind the feature flag runner_instructions, rollout issue: #296818 (closed)

#214587 (closed)

Screenshots (strongly suggested)

Show modal button	Linux/macOS/Windows	Docker/Kubernetes

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