gnu: Add hplip-minimal.

* gnu/packages/cups.scm (hplip-minimal): New variable.
* gnu/packages/scanner.scm (sane-backends)[inputs]: Replace hplip with
hplip-minimal.
* doc/guix.texi (Printing Services): Change example code to use
hplip-minimal in place of hplip. Add note explaining when use hplip.