Add missing descriptions, and move reference index page

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

engine/api/sdk/examples.md

@@ -1,5 +1,6 @@
 ---
 title: Examples using the Docker Engine SDKs and Docker API
+description: Examples on how to perform a given Docker operation using the Go and Python SDKs and the HTTP API using curl.
 keywords: developing, api, sdk, developers, rest, curl, python, go
 redirect_from:
 - /engine/api/getting-started/

reference/index.md

@@ -1,5 +1,6 @@
 ---
 title: Reference documentation
+description: This section includes the reference documentation for the Docker platform’s various APIs, CLIs, and file formats.
 notoc: true
 ---
 

samples/index.md

@@ -1,5 +1,6 @@
 ---
 title: Samples
+description: Learn how to develop and ship containerized applications, by walking through samples that exhibits canonical practices.
 ---
 
 {% assign labsbase = "https://github.com/docker/labs/tree/master" %}