Fix table of content links on Rest API page (#35348)

Fixes the outdated MDM link, and adds the missing top-level links.

_The notation for integrations `#integrations-1`, is to take the second
occurence of integrations, as we have another integrations title in the
update webhook configuration._

I can see this doesn't affect the website (or shouldn't) since it uses
it's own way to generate the TOC, but will help for manually browsing
the markdown.

docs/REST API/rest-api.md

@@ -6,7 +6,10 @@
 - [File carving](#file-carving)
 - [Hosts](#hosts)
 - [Labels](#labels)
-- [Mobile device management (MDM)](#mobile-device-management-mdm)
+- [OS Settings](#os-settings)
+- [Setup Experience](#setup-experience)
+- [Commands](#commands)
+- [Integrations](#integrations-1)
 - [Policies](#policies)
 - [Queries](#queries)
 - [Schedule (deprecated)](#schedule)