docs: Add documentation on how to add new tools to codeact_agent

Added comprehensive documentation about how to add new tools to the codeact_agent in its README.md file. The documentation includes:

How tools are defined using ChatCompletionToolParam
A complete example of defining a new tool
Steps required to add a new tool
List of currently supported built-in tools
Configuration parameters for enabling/disabling tools

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:869ce8f-nikolaik   --name openhands-app-869ce8f   docker.all-hands.dev/all-hands-ai/openhands:869ce8f

All-Hands-AI / OpenHands

docs: Add documentation on how to add new tools to codeact_agent #5150