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 yslt.yml2

estylesheet {
    template "/list" cities
        apply "address[name='Meier']" with "me", ".";

    template "address" {
        param "me";

        call "f" with "addresses", "$me/address[name='Schmid'] | .";
    }

    function "f" {
        param "addresses";

        foreach "$addresses"
            city value "city";
    }
}