Docs: Repoint Slack links directly to rossoctl Slack

[esnible]

Summary

Repoint Slack links to directly go to Rossoctl Slack, rather than taking advantage of the Slack redirect.

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[mrsabath]

LGTM. Repoints all four Slack references to ibm.biz/rossoctl-slack, and I confirmed the new short-link is live — it redirects to the renamed rossoctl.slack.com workspace (same invite token). All four kagenti-slack occurrences in the repo are covered; nothing left behind.

Scoped cleanly to Slack. The Google Group (kagenti-contributors) and community calendar (kagenti@kagenti.io) in these same files are still Kagenti-branded — those stay tracked under rossoctl/rossoctl#2285, no need to expand this PR.

Areas reviewed: Docs / site config (link correctness verified live). CI: DCO pass, deploy preview built. Commit: signed-off.