Add runner setup walkthrough

When we know that a user is building an IOS application we should update the first time experience to guide them through set up for what is needed in order to deploy onto IOS.

This MR adds the runner setup walkthrough.

Part of issue: Update pipeline zero state to suggest IOS speci... (gitlab-org/growth/team-tasks#509 - closed)

Screenshots or screen recordings

Small demo video of this part of the experiment: https://youtu.be/7_QqmSdL26I

On SaaS when shared runners are disabled	On SaaS when shared runners are disabled displayed on smaller screens	Modal after clicking 'Set up a runner' button	On SaaS when shared runners are enabled or after closing the modal

How to set up and validate locally

• In the console, enable the experiment: Feature.enable(:ios_specific_templates)
• Find a local project which does not have pipelines yet
• In the console, add iOS as a target platform: project.project_setting.update(target_platforms: ['ios'])
• Disable Default to Auto DevOps pipeline in Settings > CI/CD > Auto DevOps
• Visit the CI/CD > Pipelines page of the project

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.