Backport "refactor: some more scaladoc cleanup" to LTS (#18990)

Backports #17967 to the LTS branch.

PR submitted by the release tooling.
[skip ci]

Author: Kordyjan

.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")")"