Convert sum command into subcommand of the math command (#2004)

* Convert sum command into subcommand of the math command

* Add bullet points to math.md documentation

crates/nu-cli/tests/commands/merge.rs

@@ -33,7 +33,7 @@ fn row() {
                 | merge { open new_caballeros.csv }
                 | where country in: ["Guayaquil Ecuador" "New Zealand"]
                 | get luck
-                | sum
+                | math sum
                 | echo $it
                 "#
         ));