Replace remaining uses of shorthand codeblock syntax, add CI check

_tools/check-rst.sh

@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+
+set -uo pipefail
+
+output=$(grep -r -P '^(?!\s*\.\.).*\S::$' --include='*.rst' --exclude='docs_writing_guidelines.rst' .)
+if [[ -n $output ]]; then
+  echo 'The shorthand codeblock syntax (trailing `::`) is not allowed.'
+  echo "$output"
+  exit 1
+fi