Add subscription creation section to the Marketplace API docs

What does this MR do and why?

This MR introduces documentation on how to create a customer subscription and get its status using the Marketplace API.

Related issue

https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/5316

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A

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.