Add documentation for tracking by domain name

I recently wanted to track someone based on their http-proven website identity, and I had to screw around with the CLI for a bit before hitting on `keybase track http://idlewords.com`. It would be great if the CLI formats for the different identity types could be documented on the web and in the CLI for at least the `id` and `track` commands.

Start with the web documentation and CLI help for the `id` and `track` commands, then identify how the supported identity types and their formats are currently presented. Done means both locations document the formats needed to track identities by domain name, including the `http://idlewords.com` example.

Written by the indexing model from the issue text.