Auto-generate markdown command docs (#4451)

* Finish updating

* a couple improvements

* Update renames

* cleanup examples

docs/commands/let-env.md

@@ -1,16 +1,17 @@
-# let-env
+---
+title: let-env
+layout: command
+version: 0.59.0
+---
+
 Create an environment variable and give it a value.
 
-## Usage
-```shell
-> let-env <name> <equals> <expr> {flags} 
- ```
+## Signature
+
+```> let-env (var_name) (initial_value)```
 
 ## Parameters
-* `<name>` the name of the environment variable
-* `<equals>` the equals sign
-* `<expr>` the value for the environment variable
 
-## Flags
-* -h, --help: Display this help message
+ -  `var_name`: variable name
+ -  `initial_value`: equals sign followed by value