docs: Include files are always searched at standard location, too.

This removes the need to specify the location where yml2 was unpacked or
installed as YML_PATH. So this now works:

YML_PATH=/foo ./yml2proc --xml2yml

The standard system location <prefix>/share/yml2 is now always appended to
the include search path. If this path does not exist, the directory one
above the `backend.py` file is added, allowing the same behaviour for
uninstalled yml2.

In the Makefile enforce the .ysl2 files to be searched in the current
directory first.

include uml.yml2

UML {
    include uml2idl.yml2

    Root apply "packagedElement", 0;

    Package {
        | module «@name» {
        applyClass;
        | };
    }

    Class("@stereotype='Struct'") {
        | interface «@name» {
        applyAttributes;
        | };
    }

    Attribute | attribute †type «@name»;
}