Add search engine setup documentation

[xingyaoww]

• 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.

Adds documentation on how to set up the search engine functionality using Tavily API in OpenHands. This helps users understand how to enable web search capabilities for their agents.

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

This PR adds:

1. A new documentation page under "Advanced Configuration" that explains how to set up and use the Tavily search engine with OpenHands
2. Updates the installation documentation to include a section about setting up the search engine
3. Adds different instructions for self-hosted OpenHands vs. OpenHands Cloud

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Link of any specific issues this addresses:

N/A

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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:e983ac5-nikolaik   --name openhands-app-e983ac5   docker.all-hands.dev/all-hands-ai/openhands:e983ac5

[neubig]

LGTM, thanks for adding!