Add delivery channels list to documentation

I can't find a list of supported delivery channel types [here](https://moira.readthedocs.io/en/latest/user_guide/subscriptions.html) or anywhere else in the documentation. The most detailed info is contained on a screenshot (sic!) on the [GSoC page](https://moira.readthedocs.io/en/latest/gsoc.html).

I suggest adding a list of supported delivery channel types with descriptions.