Document a few strategies to extract EE APIs

What does this MR do?

Document a few strategies to extract EE APIs

Preview at https://gitlab.com/gitlab-org/gitlab-ce/blob/docs-extract-ee-api/doc/development/ee_features.md#code-in-libapi

Why was this MR needed?

Those strategies were used in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4404 and https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4854

We would add more or update it as we go.

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Review
  • Has been reviewed by Backend

What are the relevant issue numbers?

Part of https://gitlab.com/gitlab-org/gitlab-ee/issues/4643, for Grape APIs.