[Docs]: add slack integration docs

[malhotra5]

• This change is worth documenting at https://docs.all-hands.dev/
• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

End-user friendly description of the problem this fixes or functionality this introduces.

• Docs outline cloud openhands integration with slack

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Summarize what the PR does, explaining any non-trivial design decisions.

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Link of any specific issues this addresses:

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To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:6e491c7-nikolaik   --name openhands-app-6e491c7   docker.all-hands.dev/all-hands-ai/openhands:6e491c7