feat: add /llms.txt endpoints for LLM-friendly documentation

[claw-explorer]

Summary

Adds two new routes that serve LLM-friendly plain text documentation following the llms.txt specification:

• GET /llms.txt — Returns a top-level index of all available documentation with links to each doc section.
• GET /:doc/llms.txt — Returns a per-documentation index with all entries grouped by type, linking to individual pages.

Format

The output follows the llms.txt standard:

# DevDocs

> DevDocs combines multiple API documentations in a fast, organized, and searchable interface.

## Documentation

- [CSS](https://devdocs.io/css/)
- [JavaScript](https://devdocs.io/javascript/)
...

Per-doc endpoints group entries by type:

# JavaScript

> JavaScript documentation on DevDocs.

## Array

- [Array.from()](https://devdocs.io/javascript/global_objects/array/from)
- [Array.isArray()](https://devdocs.io/javascript/global_objects/array/isarray)
...

## Promise

- [Promise.all()](https://devdocs.io/javascript/global_objects/promise/all)
...

Why

LLM tools and AI-powered development environments need efficient, structured access to documentation. The llms.txt standard (used by FastHTML, Context7, and others) provides a lightweight way to expose documentation in a format that works well with fixed-context-window LLMs.

Implementation

• Routes are defined before the catch-all doc route to avoid conflicts
• Uses existing settings.docs manifest and per-doc index.json files
• No new dependencies required
• Served as text/plain with Markdown formatting

Closes #2520