Add documentation for building programs using AssemblyScript. (#1782)

* Add documentation for building programs using AssemblyScript.

* Add the assemblyscript hello world as an example and display it inline.

* Move the AssemblyScript hello world into the docs directory.

That way Cargo doesn't try to run it like a Rust example.

docs/assemblyscript-hello-world/wasi-hello-world.ts

@@ -0,0 +1,5 @@
+import "wasi"
+
+import {Console} from "as-wasi"
+Console.log('Hello World!\n');
+