docs: add missing channel type

- `tool` - tool usage by trainees will be sent to the channel with this type.

  Injects _cannot_ be specified with this type and there can be at most _one channel_ with this type in a definition

- `email` - injects meant to represent email communication

- `form` - questionnaires will be sent to this channel

_At most one channel_ of a specific type can exist.

In other words, there cannot be multiple channels with type `email`.

If a definition does not contain a channel with the `tool` type, _no tools_ can be specified.

Same rule applies to emails.

Same rule applies to emails and questionnaires.

When a channel of some type is specified, at least one related object must be specified.

E.g. if an `email` channel exists, at least one email address must be specified in `emails.yml`.