Exclude doc/cprover-manual from Doxygen input

The documentation in the folder doc/cprover-manual was not intended to
be included in the Doxygen documentation.  It is the source for a
separate website.

Author: John Nonweiler

src/doxyfile

@@ -813,7 +813,8 @@ RECURSIVE              = YES
 # Note that relative paths are relative to the directory from which doxygen is
 # run.
 
-EXCLUDE                = solvers/z3/
+EXCLUDE                = solvers/z3/ \
+                         ../doc/cprover-manual
 
 # The EXCLUDE_SYMLINKS tag can be used to select whether or not files or
 # directories that are symbolic links (a Unix file system feature) are excluded