WSUG: Convert the "Capturing" chapter to AsciiDoc.

Leave most of the content intact for now. Add a section on markup
conventions to README.txt. Remove the .mediaobject border.

Change-Id: I06cfd482a4c8ea63c90b9f59fcdf2afaf636a4a2
Reviewed-on: https://code.wireshark.org/review/3821
Reviewed-by: Gerald Combs <gerald@wireshark.org>