Auto-generate markdown command docs (#4451)

* Finish updating

* a couple improvements

* Update renames

* cleanup examples

docs/commands/path_exists.md

@@ -0,0 +1,28 @@
+---
+title: path exists
+layout: command
+version: 0.59.0
+---
+
+Check whether a path exists
+
+## Signature
+
+```> path exists --columns```
+
+## Parameters
+
+ -  `--columns {table}`: Optionally operate by column path
+
+## Examples
+
+Check if a file exists
+```shell
+> '/home/joe/todo.txt' | path exists
+```
+
+Check if a file exists in a column
+```shell
+> ls | path exists -c [ name ]
+```
+