doc: Fix Slack community invite link

[nejraselimovic]

Summary

Closes #532

Checklist

• [x] Scope: The referenced issue is addressed, there are no unrelated changes.
• [x] Run Locally: Verified that the docs build using make server, posted screenshots, verified external links.
• [x] Style Guidelines: Documentation obeys style guidelines in CONTRIBUTING.
• [x] Commits: Commit messages follow guidelines in CONTRIBUTING, there are no fixup commits left.