Auto-generate markdown command docs (#4451)

* Finish updating

* a couple improvements

* Update renames

* cleanup examples

docs/commands/dfr_count-null.md

@@ -0,0 +1,20 @@
+---
+title: dfr count-null
+layout: command
+version: 0.59.0
+---
+
+Counts null values
+
+## Signature
+
+```> dfr count-null ```
+
+## Examples
+
+Counts null values
+```shell
+> let s = ([1 1 0 0 3 3 4] | dfr to-df);
+    ($s / $s) | dfr count-null
+```
+