docs: Move tutorials/documenting-api-endpoints to documentation/api.

Move docs/tutorials/documenting-api-endpoint.md to
docs/documentation/api.md.

This makes it easier to find when browsing the complete set of
materials on writing Zulip documentation.

docs/documentation/openapi.md

@@ -1,4 +1,4 @@
-# OpenAPI REST API documentation
+# OpenAPI configuration
 
 The [OpenAPI](http://swagger.io/specification/) (formerly known as
 Swagger) specification is a standardized way to describe how an API