doc: Update to latest discord invite link

[brian-intel]

PR Checklist

• Added label to the Pull Request for easier discoverability and search
• Commit Message meets guidelines as indicated in the URL https://github.com/intel-retail/automated-self-checkout/blob/main/CONTRIBUTING.md
• PR introduces changes that breaks compatibility with other modules (If YES, please provide details below)
• No Employer Confidential details are added

What requirement is this design document for?

Update to latest discord invite link

#issue_number

Anything the reviewer should know when reviewing this PR?

If there are any other design Pull Requests or requirements, please link them here (i.e. intel-retail/automated-self-checkout )