Add /api endpoint

[matthewhanson]

Add /api endpoint to return the OpenAPI spec for this API.

[matthewhanson]

We can create a new yaml file based on
https://github.com/radiantearth/stac-spec/blob/master/api-spec/STAC-extensions.yaml

Follow instructions here
https://github.com/radiantearth/stac-spec/blob/master/api-spec/README.md#openapi-definitions

to create an API doc with core + all extensions but without the transaction extension.