docs: use H4 for API entries, H3 for files

docs/gen_api_ref_v2.py

@@ -114,7 +114,7 @@ def to_md(doc, sig):
     if not brief:
         return ''
     
-    md = f"### {brief}\n\n"
+    md = f"#### {brief}\n\n"
     
     # Clean signature
     clean = sig.strip().rstrip(';')