Add headings to hook pages to keep "Summary" short

The paragraph between the "Summary" heading and the next heading needs
to be short because we use this paragraph when generating the hooks
summary page.

docs/hooks/hook_civicrm_alterMenu.md

@@ -4,6 +4,8 @@
 
 This hook is called when building CiviCRM's list of HTTP routes and should be used when you want to register custom paths or URLS.
 
+## Notes
+
 !!! note "Comparison of Related Hooks"
     This is one of three related hooks. The hooks:
 

docs/hooks/hook_civicrm_xmlMenu.md

@@ -5,6 +5,8 @@
 This hook is called when building CiviCRM's menu structure, which is
 used to render urls in CiviCRM.
 
+## Notes
+
 !!! note "Comparison of Related Hooks"
     This is one of three related hooks. The hooks: