Auto-generate markdown command docs (#4451)

* Finish updating

* a couple improvements

* Update renames

* cleanup examples

docs/commands/keep_while.md

@@ -0,0 +1,23 @@
+---
+title: keep while
+layout: command
+version: 0.59.0
+---
+
+Keep elements of the input while a predicate is true.
+
+## Signature
+
+```> keep while (predicate)```
+
+## Parameters
+
+ -  `predicate`: the predicate that kept element must not match
+
+## Examples
+
+Keep while the element is negative
+```shell
+> echo [-1 -2 9 1] | keep while $it < 0
+```
+