Auto-generate markdown command docs (#4451)

* Finish updating

* a couple improvements

* Update renames

* cleanup examples

docs/commands/extern.md

@@ -0,0 +1,17 @@
+---
+title: extern
+layout: command
+version: 0.59.0
+---
+
+Define a signature for an external command
+
+## Signature
+
+```> extern (def_name) (params)```
+
+## Parameters
+
+ -  `def_name`: definition name
+ -  `params`: parameters
+