btrfs-progs: build: add support for asciidoctor doc generator

We've been using asciidoc that's written in python2, which is going to
be phased out and deprecated next year. There's a replacement,
asciidoctor. Add a configure-time detection which tool is available,
update Documentation/Makefile.in.

The original asciidoc tool is still preferred as it produces slightly
better output. The file asciidoc.conf does not have a direct equivalten
in asciidoct and would need to be replaced by extension written in ruby.

The differences:

- the <literal> are not automatically underlined and are less visible in
  the generated manual page, but it's still acceptable
- the inline CSS for the html output looks subjectively worse, is less
  compact and colourful

Issue: #89
Signed-off-by: David Sterba <dsterba@suse.com>

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