Add documentation for chat and killfeed API endpoints

[Bash-09]

Needs to be added to https://github.com/MegaAntiCheat/client-backend/wiki/API-Specification

New endpoints are GET mac/chat/v1 and GET mac/killfeed/v1

Structure of returned killfeed data may be subject to change in the near future (at time of writing, if suicide support has been added already then it's probably fine), but chat should be pretty stable.

Probably easiest to just play a test match for a little bit, query it yourself and look at the structure of the data. I'm hoping it's already pretty decently structured.