Adds final answer tool for all agents

[aymeric-roucher]

What does this PR do?

Fixes a few errors reported by @MoritzLaurer .

• Fixes typo in doc
• Explains agent.logs in doc

Important change:
The CodeAgent would not always return the right output, because it would always only return the last element, regardless of wether the important element to return was the last one or not.

For instance, when asked to generate an image, the generated code could look like:

image = image_generator("Draw me a lamb")
print("Image correctly generated!")

Which returned string "Image correctly generated!" instead of the expected image.

To fix this, I just make all agents use the final_answer tool: thus the agent can return one specific outcome of its run by passing it to final_answer.
This also simplifies the code!

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[LysandreJik]

Cool! Thanks @aymeric-roucher