Point Growth Slack references at #s_growth

Why is this change being made?

Growth is consolidating its three per-group Slack channels — #g_acquisition, #g_activation, #g_engagement — into the stage channel #s_growth. With the team operating in a pooled model the per-group channels fragment discussion without a current need, so they are being de-referenced everywhere and then archived. The change is reversible: if per-group channels are needed again they can be recreated and this reverted.

This MR updates the two hand-written handbook pages that name those channels:

• content/handbook/engineering/development/growth/_index.md — the "Teams & Communication" section. The Slack channel moves out of the table and sits above it as a single link, since all four rows would otherwise repeat the same value. The table narrows to Team + GitLab Handle.
• content/handbook/marketing/growth/_index.md — the Acquisition team's Contact block.

The GitLab Handle values are unchanged. The three groups remain distinct groups with their own GitLab groups, group::* labels, and error budgets — only the Slack channel changes.

The auto-generated category pages (/handbook/product/categories/, .../features/, .../lookup/) render the channel from data/stages.yml in www-gitlab-com, which the handbook syncs at build time. Those are covered by a separate MR, linked below, and need no change here.

Verification

Run locally against this branch:

• markdownlint-cli2 on both files — Summary: 0 error(s).
• vale on both files — 0 errors. Warning count is identical with and without this change (8 on the engineering page), so it introduces no new findings; the existing ones are surnames and "signup"/"Saas" spellings on untouched lines.
• grep -rn 'g_acquisition\|g_activation\|g_engagement' content/ — no matches remain.

Not run locally: the hugolint linkcheck job, which runs from a container image in CI.

Author and Reviewer Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
• Assign reviewers for this MR to the correct DRI
  • CODEOWNERS covers both paths: @jeromezng for /content/handbook/engineering/development/growth/, and @gdoud @manav-gl for /content/handbook/marketing/growth/.
• For transparency, share this MR with the audience that will be impacted.
  • Team: share in #s_growth
  • Department: not applicable — this affects the Growth stage only
  • Division: not applicable
  • Company: not applicable

References

• Source issue: gitlab-org/growth/team-tasks#1012
• Canonical group data (data/stages.yml): gitlab-com/www-gitlab-com!144004 (merged)
• Error budget report routing (services/teams.yml): gitlab-com/runbooks!11225 (merged)

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