Add a case_conflict.py to check for case-only file conflicts.

This is confusing and can cause issues for folks running case
insensitive filesystems (happens in windows sometimes).

If there is a conflict the pages in question are output. And you get an
non-zero exit status. E.g.

    Conflict!
            ../../README.md
            ../../Readme.md

.github/scripts/case_conflicts.py

@@ -0,0 +1,27 @@
+from collections import defaultdict
+from glob import glob
+
+def main() -> None:
+    found_conflict = False
+    all_markdown_paths = glob("../../*.md") + glob("../../**/*.md")
+
+    paths_by_lower_path = defaultdict(list)
+    for path in all_markdown_paths:
+        paths_by_lower_path[path.lower()].append(path)
+
+    for lower_path, original_paths in paths_by_lower_path.items():
+        #print(f"count={len(original_paths)} - {lower_path}")
+        if len(original_paths) == 1:
+            # No conflict.
+            continue
+
+        found_conflict = True
+        print("Conflict!")
+        for path in original_paths:
+            print(f"\t{path}")
+
+    if found_conflict:
+        exit(1)
+
+if __name__ == '__main__':
+    main()