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/contributing/gsod-ideas.md

@@ -211,7 +211,7 @@ are clearly correct and that explain why they are correct.
 
 Fill in the gaps in Zulip's
 [REST API documentation](https://zulipchat.com/api).  Zulip has a
-[nice framework](../tutorials/documenting-api-endpoints.html) for
+[nice framework](../documentation/api.html) for
 writing API documentation built by a student last summer based on the
 OpenAPI standard with built-in automated tests, but there are dozens of
 endpoints that are missing, several of which are quite important.  See

docs/documentation/index.rst

@@ -8,5 +8,5 @@ Writing Documentation
    overview
    user
    integrations
+   api
    openapi
-

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

docs/documentation/overview.md

@@ -104,7 +104,7 @@ To learn more about Zulip's integrations documentation,
 
 To learn more about Zulip's API documentation,
 [visit it on zulipchat.com](https://zulipchat.com/api/) or
-[read our tutorial on writing user documentation](../tutorials/documenting-api-endpoints.html).
+[read our tutorial on writing user documentation](../documentation/api.html).
 
 ## Automated testing
 

docs/overview/gsoc-ideas.md

@@ -260,7 +260,7 @@ CSS](https://github.com/zulip/zulip/).
 
 - Fill in the gaps in Zulip's
   [REST API documentation](https://zulipchat.com/api).  Zulip has a
-  [nice framework](../tutorials/documenting-api-endpoints.html) for
+  [nice framework](../documentation/api.html) for
   writing API documentation built by a student last summer based on
   the OpenAPI standard with built-in automated tests, but there are a
   few dozen endpoints that are missing, several of which are quite

docs/tutorials/index.rst

@@ -11,4 +11,3 @@ Developer Tutorials
    reading-list
    screenshot-and-gif-software
    shell-tips
-   documenting-api-endpoints