Fix: add documentation for IHTA namespace

In the Doxygen documentation, the namespace list did not show all namespaces. This was caused by the missing documentation of the IHTA namespace, which precedes all other namespace names.

This MR adds documentation for the IHTA namespace.

The only downside is that now all occurrences of "IHTA" in the written text link to the namespace page. In my opinion, this is a reasonable drawback.