Add terraform module upload API documentation

What does this MR do and why?

In https://docs.gitlab.com/ee/api/packages/terraform-modules.html, all available endpoints to interact with the Terraform Registry should be documented. However, the upload endpoint is missing. Check this thread for more context.

In this MR, I added the Terraform Registry's upload endpoint documentation to https://docs.gitlab.com/ee/api/packages/terraform-modules.html.

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.

Related to #433097 (closed)