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.

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include homepage.en.yhtml2

     2

     3

page "Hello, world" {

     4

    p   >>

     5

        Hello, world! I can link here, say:

     6

        ¬http://en.wikipedia.org to Wikipedia¬

     7

        >>

     8

     9

    p   >>

    10

        This is ƒemphasized. And this is «code».

    11

        >>

    12

}