Add llms.txt

To help people trying to use an LLM to help them learn / use Talon, we should provide an llms.txt file that people can add as context. See:

- https://llmstxt.org/
- https://docs.anthropic.com/llms.txt
- https://docs.anthropic.com/llms-full.txt

We should obv try to generate this automatically, which shouldn't be too hard given our docs are in markdown in the first place