Guidelines to developers regarding API versioning.

For now, the API endpoints are also availble under /v1.

We need to make a /v2 version of the API when we make breaking changes, as well as figure out how best to continue supporting previous versions of the API so they can be deprecated gracefully.

• openapi info section should specify API version
• previous versions of API will be copied into a separate openapi file, eg: openapi_v1.yaml
• documentation should make it clear which path to use to reach which version of the API, and what the current version of the “unversioned” API path is
• specs should be hosted and available at /openapi/v1, /openapi/v2 endpoints
• enhance integration / e2e tests to illustrate differences between API versions