Developer docs: 5-minute quickstart and a plain-text API reference

New documentation hub at /docs. The quickstart walks from signup to a webhook-verified tracker in five steps, with copy-paste curl for every call.

The entire API reference is also published as plain text for AI tools: /llms.txt is the index, /llms-full.txt the complete reference — both generated from the same OpenAPI spec as the interactive docs, so they never drift. Claude, Cursor, and other coding agents can read the full API surface in one request.

• /docs — quickstart, API reference, MCP server, changelog
• /docs/quickstart — first tracker in ~5 minutes, including webhook signature verification
• /llms.txt and /llms-full.txt — machine-readable reference for AI coding tools
• /api/openapi — the OpenAPI 3.1 spec itself, no auth required