Add llms.txt to prompting guide, and new 'About llms.txt' page

[jason-kincaid]

Summary

This PR:

• Adds guidance in the Workers LLM Prompt to direct users/LLMs to llms.txt when needed
• Adds a new page, about-llms-txt.mdx explaining the llms.txt convention
• Updates the LLM prompting guide to reference this resource

Please cherrypick and revise as is fitting.

[github-actions]

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[hyperlint-ai]

Hyperlint Review is complete!

Hyperlint Vale Style Guide Review

Reviewed 2 files and found 1 issues.

See the inline comments and suggestions below.

Hyperlint Link Check Review

Reviewed 2 files and found 0 issues.

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[hyperlint-ai]

Identified issues

• Vale Style Guide - (cloudflare.MeaningfulLinkWords-warning) Warning: Rewrite the link text for [here](/about-llms-txt/) to be more descriptive. For more information, refer to Descriptive link text.

Proposed fix

Suggested change

	For many applications, this prompt will be all you need. You can further supplement it with the full documentation for any particular product, available in an LLM-optimized format at `developers.cloudflare.com/[product-name]/llms-full.txt`. Learn more about llms.txt [here](/about-llms-txt/).
	Learn more about llms.txt in our detailed guide [about LLMs](/about-llms-txt/).

The link text 'here' is not descriptive. I've changed it to 'about LLMs' to provide more context about the link's destination.