[Response Ops][Actions] Document how to look up channel ID for slack web API connector

The slack web API connector requires a channel ID (instead of channel name) but we've gotten feedback that people do not know how to look up the channel ID.

The docs mention channel ID: https://www.elastic.co/guide/en/kibana/master/slack-action-type.html#configuring-slack-web-api but it would be useful to add screenshots for how to look up the channel ID for a channel in slack.

As an additional step, it would be helpful to link to the updated docs from the connector form near the channel ID input