Register users_guide.pdf into doc-base

[rezib]

This will notably fix lintian information:

I: clara-core: possible-documentation-but-no-doc-base-registration
N: 
N:    The package ships a .html or .pdf file under /usr/share/doc/, which are
N:    usually documentation, but it does not register anything in doc-base.
N:    (Files under an examples directory are excluded.)
N:    
N:    Refer to Debian Policy Manual section 9.10 (Registering Documents using
N:    doc-base) for details.
N:    
N:    Severity: wishlist, Certainty: possible
N:    
N:    Check: menus, Type: binary
N: