Lots of stuff to document! New tools.md top level page.

Preview: https://github.com/simonw/llm/blob/tools/docs/tools.md

TODO:

• Document register_tools() plugin hook
• Basic explanation of what tools are and how they work in LLM
• Python API for registering tools with a prompt (function and llm.Tool) - including note on how async works
• How to use tools with llm -T x r llm --tool X (link this from the usage page too)
• Docs and example of --python-tools usage for the CLI
• Also mention llm tools and llm tools --python-tools debug mechanism
• Python API for running a prompt with tools via the .chain() method
• The debugging hooks before_call and after_call
• Documentation on how to implement tool usage for model plugins #1000
• Short tutorial on using https://github.com/simonw/llm-plugin-tools cookiecutter template