docs: Add ai.txt, llms.txt, and llms-full.txt for LLM discovery

[qkraudghgh]

Summary

• Add /ai.txt, /llms.txt, and /llms-full.txt at the docs site root, following the llmstxt.org and aitxt.ing standards, so LLM crawlers and agents can index overlay-kit's public API directly without scraping the rendered Nextra site.
• llms.txt is a navigable index of guides / API / design-system integrations. ai.txt carries canonical facts and "do not hallucinate" guardrails. llms-full.txt inlines the core English docs; the marketing landing, verbose test code in the Testing guide, and the per-design-system walkthroughs (MUI / Chakra UI / Ant Design) are intentionally excluded as noise (see the file header).
• Exempt the three text files from the nextra/locales i18n middleware so they respond at the site root (/llms.txt) instead of being 307-redirected to /en/llms.txt — required by both standards.

Test plan

• yarn dev then curl http://localhost:3939/ai.txt → HTTP 200, text/plain; charset=UTF-8, 4050 B
• yarn dev then curl http://localhost:3939/llms.txt → HTTP 200, text/plain; charset=UTF-8, 5959 B
• yarn dev then curl http://localhost:3939/llms-full.txt → HTTP 200, text/plain; charset=UTF-8, 27246 B
• After deploy, verify https://overlay-kit.slash.page/{ai,llms,llms-full}.txt are reachable at the site root with no /en redirect.

🤖 Generated with Claude Code

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Latest commit: 5f7ecce

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.85%. Comparing base (4619b2f) to head (5f7ecce).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #225   +/-   ##
=======================================
  Coverage   99.85%   99.85%           
=======================================
  Files          15       15           
  Lines        1340     1340           
  Branches      262      262           
=======================================
  Hits         1338     1338           
  Misses          2        2           

Components	Coverage Δ
overlay-kit	99.85% <ø> (ø)

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