Auto-generate markdown command docs (#4451)

* Finish updating

* a couple improvements

* Update renames

* cleanup examples

docs/commands/columns.md

@@ -0,0 +1,29 @@
+---
+title: columns
+layout: command
+version: 0.59.0
+---
+
+Show the columns in the input.
+
+## Signature
+
+```> columns ```
+
+## Examples
+
+Get the columns from the table
+```shell
+> [[name,age,grade]; [bill,20,a]] | columns
+```
+
+Get the first column from the table
+```shell
+> [[name,age,grade]; [bill,20,a]] | columns | first
+```
+
+Get the second column from the table
+```shell
+> [[name,age,grade]; [bill,20,a]] | columns | select 1
+```
+