Add OpenAPI documentation for Branches API

What does this MR do and why?

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/376870

• Add field documentation for Grape::Entity
• Add response codes for API endpoints

Please note that

• Changes to doc/api/openapi/openapi_v2.yaml were excluded to avoid conflicts within the same documentation effort.
• Errors related to consumes property were ignored as per https://gitlab.com/gitlab-org/gitlab/-/issues/377941.

How to set up and validate locally

1. Run rake gitlab:openapi:generate
2. Review changes in doc/api/openapi/openapi_v2.yaml
3. Paste the content into https://editor.swagger.io/ and look for merge request section and review the content

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.