[docs] Use yaml.safe_load() in build_*.py scripts (#2432)

Co-authored-by: itchyny <itchyny@cybozu.co.jp>

docs/build_website.py

@@ -16,7 +16,7 @@ env = Environment(
 
 def load_yml_file(fn):
   with open(fn) as f:
-    return yaml.load(f, Loader=yaml.Loader)
+    return yaml.safe_load(f)
 
 env.filters['search_id'] = lambda input: input.replace(r'`', '')
 env.filters['section_id'] = lambda input: re.sub(r"[^a-zA-Z0-9_]", '', input)