Provide llms.txt and llms-full.txt on the doc site

[doggy8088]

This idea comes from the Angular.dev site. See these two links:

1. https://angular.dev/llms.txt
2. https://angular.dev/llms-full.txt

I wish the https://lit.dev/ site could provide this feature. It would be beneficial for developers who can use AI to answer questions related to Lit.