/api: Add an overview doc for the REST API.

templates/zerver/api/rest.md

@@ -0,0 +1,25 @@
+# The Zulip REST API
+
+The Zulip REST API powers the Zulip web and mobile apps, so anything
+you can do in Zulip, you can do with Zulip's REST API.  To use this API:
+
+* You'll need to [get an API key](/api/api-keys).  You will likely
+  want to create a bot, unless you're using the API to interact with
+  your own account (e.g. exporting your personal message history).
+* Choose what language you'd like to use.  You can download the
+  [Python or JavaScript bindings](/api/installation-instructions), or
+  just make HTTP request with your favorite programming language.  If
+  you're making your own HTTP requests, you'll want to send the
+  appropriate HTTP Basic Authentication headers; see each endpoint's
+  `curl` option for details on the request format.
+* The Zulip API has a standard
+  [system for reporting errors](/api/rest_error_handling).
+
+Most other details are covered in the documentation for the individual
+endpoints:
+
+{!rest_endpoints.md!}
+
+Since Zulip is open source, you can also consult the
+[Zulip server source code](https://github.com/zulip/zulip/) as a
+workaround for how to do anything not documented here.

templates/zerver/api/sidebar.md

@@ -16,6 +16,7 @@
 
 ## REST API
 
+* [Overview](/api/rest)
 * [Installation instructions](/api/installation-instructions)
 * [API keys](/api/api-keys)
 * [Error handling](/api/rest_error_handling)