[Suggestion]: add llms.txt

[OfficialSerge]

Summary

Many frameworks are adopting new llms.txt pages in their documentation, react has yet to adopt this in their own documentation.

• see baseUI
• see shadcn

Page

https://react.dev/reference/react

Details

We should make a llms.txt route in the react docs so that devs have something to point their AI agents out in the wild to for reference with the latest react standards.

[MaxwellCohen]

Lllms.txt was added about 2 months ago, thanks to @rickhanlonii pr #8270

You can see it here: https://react.dev/llms.txt