doc: Convert the posix translator documentation to markdown

Change-Id: I6b6c3b2063c81e8799c3273434119dd9bff1be79
Signed-off-by: Lubomir Rintel <lubo.rintel@gooddata.com>
Reviewed-on: http://review.gluster.org/5486
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Vijay Bellur <vbellur@redhat.com>