Auto-generate markdown command docs (#4451)

* Finish updating

* a couple improvements

* Update renames

* cleanup examples

docs/commands/str_collect.md

@@ -0,0 +1,28 @@
+---
+title: str collect
+layout: command
+version: 0.59.0
+---
+
+creates a string from the input, optionally using a separator
+
+## Signature
+
+```> str collect (separator)```
+
+## Parameters
+
+ -  `separator`: optional separator to use when creating string
+
+## Examples
+
+Create a string from input
+```shell
+> ['nu', 'shell'] | str collect
+```
+
+Create a string from input with a separator
+```shell
+> ['nu', 'shell'] | str collect '-'
+```
+