Hint avaiblity of /help on Duo Chat splash screen

Problem to solve

Once #462122 (closed) is implemented, hinting/suggesting to try it could be a path for users to learn about Duo Chat's capabilities. The Duo Chat splash screen splash screen could be a natural place for this interaction.

Proposal

• Replace the last of those four pre-written questions with the /help command, so that users can discover this important slash command.
• Only implement this change once New help command in GitLab Duo Chat (#462122 - closed) • Mark Chao • 17.7 • At risk is done, otherwise we will motive users to use a slash command that does not exist, yet.

Action Items

These predefined prompts are defined in the client-side code (“glue code”) of every Duo Chat Host system. We need to ensure these updates remain backwards compatible with the API version the host system is using.