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

decl ip(%value, name="ip", select="'%value'") alias xsl:variable;
decl rechner(%name, *ip, test="@name = '%name'") alias xsl:when {
    ip *ip;
    | ip ist «$ip»
};

tstylesheet template "/rechner" {
    choose {
        rechner "R1", "192.168.1.10";
        rechner "R2", "192.168.1.11";
    }
}