The creation of the doxygen documentation is turned off by default -- the recreation at every new compilation takes too much time in the usual development.
A switch need to be present for the cmake command, the variable
{{{
INSTALL_DOC
}}}
is by default '''NO'''.
Use the '''-D''' option while invoking the cmake command to set it differently.
In the usual invocation of cmake in the build directory (with your appropriate ILCSoft.cmake file) this will look like
{{{
cmake -C ${YOUR_ILCSOFT_CMAKE} -D INSTALL_DOC=YES ..
}}}

Now invoke 
{{{
make doc
}}}
and the doxygen documentation will be generated in the ./doc directory of the project directory.

Use any html browser to take a look at it, starting with 
{{{
doc/html/index.html
}}}