manager: Add <see-also> links between related events

This patch adds some see-also references between related AMI events. It
focuses primarily on those events that are guaranteed to come in pairs,
such as DTMFBegin/DTMFEnd, as well as those that occur during the life
cycle of an Asterisk channel, such as Newchannel/Hangup.

Change-Id: Iaab600477052018d0f8c03d0c624c0856e9ff1f3