docs: add LLMs page and fix LLM copy button issues

[kelsos]

Summary

• Add a dedicated "Using LLMs" page documenting available LLM-friendly endpoints (llms.txt, llms-full.txt, per-page .md) with usage instructions for Cursor, Claude Code, and other tools.
• Fix root page "Copy page for LLMs" returning 404 by setting excludeIndexPage: false so index.md is generated.
• Add <llm-only> / <llm-exclude> tags to index.md so the <NavCards> Vue component is replaced with plain markdown links in the LLM output.

Test plan

• Build passes (pnpm build)
• Visit /llms page and verify it renders correctly
• Click "Copy page for LLMs" on the root page and verify markdown is returned (not 404 or HTML)
• Verify /llms.txt and /llms-full.txt still generate correctly
• Verify the LLMs page itself is excluded from llms.txt