Add details fields to new runner layout

What does this MR do and why?

☝ Depends on !110694 (merged).

This change adds the runner form fields to the runner creation form, so users can create their runner with a new workflow.

Designs: #383139[Runner-registration-page.png]

Notes:

• ⚠ Please note that this does not yet include the submit action, that will be done in the next MRs, which will include a backend integration to actually save the runner
• ✏ The font color of the descriptions above the fields will be updated separately at: gitlab-ui!3273 (merged)

Screenshots or screen recordings

How to set up and validate locally

1. Enable the feature flag: in your console rails c, and then Feature.enable(:create_runner_workflow)
2. Visit Admin -> CI/CD -> Runners
3. Confirm the "New instance runner" button is available on the top-right corner
4. Select the button to go to the new page

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 #383139 (closed)