docs: Slack user group mentions and @handle matching

[EladBarkay]

Documents CLI Slack mention changes going out in 0.26.1 (CORE-1565):

• Adds the optional usergroups:read scope to the Slack token setup scopes (Notify Slack user groups from owner and subscriber @handles #2383).
• Adds a Tagging owners and subscribers on Slack section to the OSS alerts configuration page. It explains how @handles are matched (email prefix, then display name, then user group), member vs. guest precedence, and what happens to unmatched handles (Resolve @handle owner/subscriber mentions in Slack web integration #2371, Notify Slack user groups from owner and subscriber @handles #2383).

The new section is on the OSS page, not in snippets/alerts/owner.mdx / subscribers.mdx, because those snippets are shared with the Cloud docs and this matching is CLI behavior.

Merge only after CLI 0.26.1 is published.

🤖 Generated with Claude Code

[linear]

CORE-1565

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