Auto-generate markdown command docs (#4451)

* Finish updating

* a couple improvements

* Update renames

* cleanup examples

docs/commands/export_def.md

@@ -0,0 +1,18 @@
+---
+title: export def
+layout: command
+version: 0.59.0
+---
+
+Define a custom command and export it from a module
+
+## Signature
+
+```> export def (name) (params) (block)```
+
+## Parameters
+
+ -  `name`: definition name
+ -  `params`: parameters
+ -  `block`: body of the definition
+