[Discord] Fix initial setup DM

The DM that the Discord bot first sends out currently is missing information and includes to a bad link. This DM should introduce HCB and include instructions on how to link the bot and add teammates to the HCB org through invite links.

Start at the Discord bot code that builds the initial setup DM and inspect the current message and link. Update the DM so it introduces HCB, explains how to link the bot, and explains how to add teammates through HCB organization invite links; verify the link and all instructions are present in the generated message.

Written by the indexing model from the issue text.