Auto-generate markdown command docs (#4451)

* Finish updating

* a couple improvements

* Update renames

* cleanup examples

docs/commands/str_substring.md

@@ -0,0 +1,44 @@
+---
+title: str substring
+layout: command
+version: 0.59.0
+---
+
+substrings text
+
+## Signature
+
+```> str substring (range) ...rest```
+
+## Parameters
+
+ -  `range`: the indexes to substring [start end]
+ -  `...rest`: optionally substring text by column paths
+
+## Examples
+
+Get a substring from the text
+```shell
+>  'good nushell' | str substring [5 12]
+```
+
+Alternatively, you can use the form
+```shell
+>  'good nushell' | str substring '5,12'
+```
+
+Drop the last `n` characters from the string
+```shell
+>  'good nushell' | str substring ',-5'
+```
+
+Get the remaining characters from a starting index
+```shell
+>  'good nushell' | str substring '5,'
+```
+
+Get the characters from the beginning until ending index
+```shell
+>  'good nushell' | str substring ',7'
+```
+