DOC: config: add example for server "track" keyword

DOC: config: add example for server "track" keyword

As requested on GH #2325, "track" server keyword could benefit from a
simple config example to show how to make use of it.

That's what we're doing in this commit, thanks to GH user @HAkmiller
for the suggestion.

(cherry picked from commit 48545113f4db86ad43a99ba01742a9811410e6dc)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit d270bc5864a3cec0b5faf40ae8ca9db0a365da81)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>