Add Marketplace API overview page

What does this MR do and why?

For issue https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/5313+

Create Marketplace API overview page.

How to set up and validate locally

1. Follow the GDK document to view this page
2. Go to gitlab-docs project, check out branch qzhao-add-cdot-marketplace-api-overview-page(gitlab-docs!3470 (merged))
3. Visit http://127.0.0.1:3005/ee/development/partner_marketplace/

The current version looks like

The sequence diagram looks like:

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.