Add llms.txt describing TAP and indexing every TAIP

[pelle]

Summary

Adds an llms.txt at the site root so LLM tooling can discover what TAP is and pull each TAIP's authoritative Markdown source directly.

The file is rendered by Jekyll/Liquid from each TAIP's frontmatter, so it stays current automatically as new TAIPs are added or status fields change.

Structure

• H1 + summary — what TAP is in one paragraph, plus a "Key concepts" list (agents, parties, transaction message types, authorization flow, identity/compliance, transport).
• Core resources — links to overview, message reference, transaction types, authorization flow, parties, agents, developer resources, JSON Schemas, test vectors, and the TypeScript SDK.
• TAIPs — every TAIP grouped by status (Final / Last Call / Review / Draft), each with its description from frontmatter and a link to the raw Markdown source on GitHub.
• Optional — pointers to the GitHub repo, contributing guide, and tap.rsvp marketing site.

Also in this PR

• Sets site.url: "https://taips.tap.rsvp" in _config.yml so absolute_url produces correct production URLs in llms.txt and the existing JSON feed (was previously empty, defaulting to whatever Jekyll picked).

Test plan

• After deploy, fetch https://taips.tap.rsvp/llms.txt and confirm it returns plain Markdown (Content-Type text/plain) with all 20 TAIPs grouped by status
• Spot-check a few raw markdown links resolve, e.g. https://raw.githubusercontent.com/TransactionAuthorizationProtocol/TAIPs/main/TAIPs/taip-3.md
• Confirm absolute URLs in JSON feed (/feed.xml) point to https://taips.tap.rsvp/...
• When a new TAIP lands or its status flips, regenerate the build and verify llms.txt updates without further code changes

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