[DOC] add configuration samples

configuration.txt is thorough and accurate but lacked sample configurations
clarifying both the syntax and the relations between global, defaults,
frontend, backend and listen sections. Besides, almost all examples to be found
in haproxy-en.txt or online tutorials make use of the 'listen' syntax while
'frontend/backend' is really the one to know about.
(cherry picked from commit 01ac10ad189b11c563eeb835733fba58e6c5271d)
1 file changed