Update on llms.txt generation in ADK docs

[koverholt]

Thanks for this extension! ❤️ I wanted to give you a heads up about this PR: google/adk-docs#1023.

Once that PR is merged, llms.txt will move from the adk-docs git repo to be served in the docs root instead at:

https://google.github.io/adk-docs/llms.txt

So you should be able to update the URL in this extension and hopefully the new autogenerated llms.txt will work better than the older manually generated version. 😁