refactor: some more scaladoc cleanup (#17967)

This gets rid of some more stuff in scaladoc that doesn't seem to be
used at all. You can see more details in each individual commit.

Author: ckipp01

.gitignore

@@ -95,3 +95,7 @@ contributors.js
 content-contributors.css
 docs/_spec/_site/
 docs/_spec/.jekyll-metadata
+
+# scaladoc related
+scaladoc/output/
+

scaladoc/.gitignore

@@ -1,5 +1,7 @@
 #!/usr/bin/env bash
 
+# NOTE that this file is used in CI
+
 MYDIR="$(dirname "$(readlink -f "$0")")"