doc: set STRIP_FROM_PATH

Set STRIP_FROM_PATH=../../ avoids having the output depend on the location of the RIOT repo in the user's fs.

doc/doxygen/riot.doxyfile

@@ -152,7 +152,7 @@ FULL_PATH_NAMES        = YES
 # will be relative from the directory where doxygen is started.
 # This tag requires that the tag FULL_PATH_NAMES is set to YES.
 
-STRIP_FROM_PATH        =
+STRIP_FROM_PATH        = ../../
 
 # The STRIP_FROM_INC_PATH tag can be used to strip a user-defined part of the
 # path mentioned in the documentation of a class, which tells the reader which