Auto-generate markdown command docs (#4451)

* Finish updating

* a couple improvements

* Update renames

* cleanup examples

docs/commands/dfr_drop-nulls.md

@@ -0,0 +1,32 @@
+---
+title: dfr drop-nulls
+layout: command
+version: 0.59.0
+---
+
+Drops null values in dataframe
+
+## Signature
+
+```> dfr drop-nulls (subset)```
+
+## Parameters
+
+ -  `subset`: subset of columns to drop nulls
+
+## Examples
+
+drop null values in dataframe
+```shell
+> let df = ([[a b]; [1 2] [3 0] [1 2]] | dfr to-df);
+    let res = ($df.b / $df.b);
+    let a = ($df | dfr with-column $res --name res);
+    $a | dfr drop-nulls
+```
+
+drop null values in dataframe
+```shell
+> let s = ([1 2 0 0 3 4] | dfr to-df);
+    ($s / $s) | dfr drop-nulls
+```
+