Use How to join community as reference for slack, discord, issues links

[mamoodi]

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

• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

Small doc update to dedup slack, discord links

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Give a summary of 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:7d2b2f9-nikolaik   --name openhands-app-7d2b2f9   docker.all-hands.dev/all-hands-ai/openhands:7d2b2f9

[neubig]

Thanks a lot!