Auto-generate markdown command docs (#4451)

* Finish updating

* a couple improvements

* Update renames

* cleanup examples

docs/commands/str_length.md

@@ -0,0 +1,28 @@
+---
+title: str length
+layout: command
+version: 0.59.0
+---
+
+outputs the lengths of the strings in the pipeline
+
+## Signature
+
+```> str length ...rest```
+
+## Parameters
+
+ -  `...rest`: optionally find length of text by column paths
+
+## Examples
+
+Return the lengths of multiple strings
+```shell
+> 'hello' | str length
+```
+
+Return the lengths of multiple strings
+```shell
+> ['hi' 'there'] | str length
+```
+