Add code Learn GitLab step

What does this MR do and why?

1. Add a section to add code within Learn GitLab that redirects to the new WebIDE
2. Tracking - we want to ensure clicks to this step in Learn GitLab is tracked

Can be reviewed by commits

Screenshots or screen recordings

How to set up and validate locally

• Export this environment variable to simulate a SaaS instance
• Sign in as root. Disable user confirmation during sign up.
• Sign out. Go to /users/sign_up and fill up the form, continue.
• Choose Just me, Create a new project on the next page.
• Fill the rest of the form and continue.
• Finish onboarding. Visit Learn GitLab page.
• Make sure the page has correct finished percentage.
• Click Add code. Update/create any file.
• Visit Learn GitLab page. Make sure Add code is marked as finished.
• Make sure the page has correct finished percentage.
• Make sure correct tracking events are present
• Click on project name. Click Web IDE. Update/create any file.
• Make sure there are no tracking events

To locally see tracking events set up: https://gitlab.com/gitlab-org/snowplow-micro-configuration#snowplow-micro

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.

Related to #384588 (closed)