Add features endpoints to OpenApi v2 docs

What does this MR do and why?

This merge request adds endpoints from API::Features to the OpenApi V2 documentation as per #211512.

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.
• Descriptions of all endpoints were updated to match the markdown documentation.
• We use multiple types for params that accept them (even if it's not supported in OpenApi V2) as per !101875 (comment 1148561131).

Resolves #376907.

Screenshots or screen recordings

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.