Added Hello World

Author: ken-matsui

pages/docs/_meta.json

@@ -2,5 +2,6 @@
     "index": "Introduction",
     "why-poac": "Why Poac?",
     "installation": "Installation",
+    "hello-world": "Hello, World!",
     "usage": "Usage"
 }

pages/docs/hello-world.mdx

@@ -0,0 +1,20 @@
+## Hello World
+
+You can get started with just a few commands as the demo shows. Let's create a new Poac project:
+
+```console
+you:~$ poac new hello_world
+     Created binary (application) `hello_world` package
+```
+
+Now, you can use the `poac run` command to run your application:
+
+```console
+you:~$ cd hello_world
+you:~/hello_world$ poac run
+ Compiling src/main.cc
+   Linking hello_world
+  Finished debug target(s) in 0.45386s
+   Running poac-out/debug/hello_world
+Hello, world!
+```