Figure out the Witness node UX

The `witness` node is now functionally available, but there are a few things that need to be addressed before exposing it.

1. A 2+1 provision option needs to be added to flyctl along with the ability to provision a witness machine.
2. We should consider adding a one-off flyctl command that allows users to add a witness node to their existing setup.
3. We need formal documentation going over the `witness` node and when/how it should be used.

**Witness machine spec**
* Same image as a normal member.
* `WITNESS` environment variable needs to be set.
* Minimal resource allocations. 1GB volume, 256mb memory, 1 cpu, etc.