This PR adds an llms.txt file that language models can consume at https://cog.run/llms.txt

It also adds an Actions workflow that:

1. Runs daily (or manually via workflow dispatch)
2. Uses the sitefetch npm package to compile the llms.txt file.
3. Commits and pushes any changes to the main branch.

I also added tips to a couple of high-traffic docs pages to make the file more discoverable to Cog model authors.