Absolute include paths in documentation.

Currently it was incorrectly stated that for using Math::Matrix or for
example Trade::MeshData it was sufficient to

    #include <Matrix.h>
    #include <MeshData.h>

which was obviously wrong, as Math/ (nor any other namespace-related
subdirectory) is not on include path. Now it states correctly that a
directory prefix is needed:

    #include <Math/Matrix.h>
    #include <Trade/MeshData.h>

The same is now fixed for classes in magnum-plugins repository.

Doxyfile

@@ -137,7 +137,8 @@ STRIP_FROM_PATH        = ../
 # definition is used. Otherwise one should specify the include paths that
 # are normally passed to the compiler using the -I flag.
 
-STRIP_FROM_INC_PATH    =
+STRIP_FROM_INC_PATH    = ../magnum/src \
+                         ../magnum-plugins/src
 
 # If the SHORT_NAMES tag is set to YES, doxygen will generate much shorter
 # (but less readable) file names. This can be useful if your file system