Improve Docs of Custom Tools and Agents
# What does this PR do?

This PR improves the docs explaining how to customize prompts and corrects some grammar, spelling, code snippets of both `transformers_agent.mdx` and `custom_tools.mdx`. Also `agent.toolbox` is made a get method / property which should help both with documentation and forbid the user to overwrite the attribute completely.

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_23255). All of your documentation changes will be reflected on that endpoint.