Auto-generate markdown command docs (#4451)

* Finish updating

* a couple improvements

* Update renames

* cleanup examples

docs/commands/dfr_is-not-null.md

@@ -0,0 +1,21 @@
+---
+title: dfr is-not-null
+layout: command
+version: 0.59.0
+---
+
+Creates mask where value is not null
+
+## Signature
+
+```> dfr is-not-null ```
+
+## Examples
+
+Create mask where values are not null
+```shell
+> let s = ([5 6 0 8] | dfr to-df);
+    let res = ($s / $s);
+    $res | dfr is-not-null
+```
+