Add more Slack channels and UX Showcase link

What problem does this MR solve?

Update the UX onboarding to have up to date information and inform team members of the UX Showcases

Solution

• Add more Slack channels
• Link to the UX showcase in the template so that new team members learn the importance of them

Checklist

1. QA check by another UX'r

/cc @gitlab-com/gitlab-ux

Relates to https://gitlab.com/gitlab-org/gitlab-design/-/issues/1084