Before this commit it is only possible to build the user guide
from the root of the repository. An invocation of "make users-guide"
in the "doc/" subdirectory fails. This commit moves the logic for
building the users-guide from the Makefile at the root of the
repository to the file "doc/Makefile".

From the root of the project only the target "users-guide"
remains, all other targets related to building the users-guide
are only available in the "doc/" subdirectory.

Fixes issue 7977