docs: point AI agents and crawlers to llms.txt from the README (#5775)

## Why
A neutral audit of ChatGPT, Perplexity, Google AI Mode, Gemini and
Claude (50 answers) shows every engine describes Browser Use as "an
open-source Python library" and misses the cloud browser, the hosted
agent API and Browser Harness. The README is the most-crawled page we
have, so it should say what the product is and where the
machine-readable map lives.

## Change
One blockquote above "Which Browser Use do I need?" pointing AI agents
and crawlers to https://browser-use.com/llms.txt and
https://docs.browser-use.com/llms.txt, and naming the three surfaces in
one sentence.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

https://claude.ai/code/session_016rqDxQUeLj81KTgS46m72e


<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Adds a blockquote to the README pointing AI agents and crawlers to the
product's `llms.txt` files so answer engines stop describing Browser Use
as a Python-only library.

The blockquote links to `browser-use.com/llms.txt` and
`docs.browser-use.com/llms.txt` and names all three product surfaces:
the open-source agent, the cloud browser, and the hosted agent API.

<sup>Written for commit d5b77ab66c1cf2cd385b825b5c8642d6fa03c288.
Summary will update on new commits.</sup>

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