Add Slack DM permissions FAQ

[NiamhRelevance]

Summary

• Adds a new FAQ entry to the Slack integration docs explaining what permissions are needed to trigger an agent via DM
• Also includes changes from prior commits: Microsoft Teams integration updates and WhatsApp migration warning

Test plan

• Verify the Slack FAQ renders correctly on the docs site
• Confirm the accordion expands and displays the permissions info

🤖 Generated with Claude Code

[linear]

TSP-982 Add FAQ about Slack DM agent trigger permission requirements

[alexrelevanceai]

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