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

tstylesheet {
    template "/list" {
        apply "address", 0;
        | 1000 FOR I=1 TO 2 STEP 1
        | 1010 READ NAME$, STREET$, VILLAGE$
        | 1020 PRINT "INSERT INTO address (name, street, village) VALUES ('" + NAME$ + "', '" + STREET$ + "', '" + VILLAGE$ + "');"
        | 1030 NEXT I
    }
    template "address" {
        | «position() * 10 + 100» DATA "«name»", "«street»", "«village»"
    }
}