All the links here are broken:
The links are not broken, or are not any longer anyway - they redirect to the Where is the DirectX SDK? page which tells us that the XACT Authoring Tool is no longer supported, that's why that part of the documentation is removed.
As AbstractChaos answered, the XACT API reference is still there though but that in turn links to the removed documentation for the tool that is posted in the question. Hence, it's removed.