docs: Update CodeAct agent documentation

[neubig]

This PR updates the documentation for the CodeAct agent to improve clarity, organization, and completeness.

Changes

• Improve overall structure and readability
• Add detailed sections for each built-in tool with key features
• Add new Overview section that better explains agent capabilities
• Add new Implementation Details section explaining core files
• Add Micro-agents section documenting specialized agents
• Reorganize content for better clarity
• Make configuration section more prominent

Files Changed

• openhands/agenthub/codeact_agent/README.md

Note

This was authored by @openhands-agent, but I (@neubig) checked and it looks OK.

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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:4c7ac72-nikolaik   --name openhands-app-4c7ac72   docker.all-hands.dev/all-hands-ai/openhands:4c7ac72