fix: serve distinct OpenAPI specs in llms.txt

[Bortlesboat]

Summary

• give the Indexer and Gateway OpenAPI sources distinct Mintlify directories
• preserve both live remote specifications instead of copying snapshots into this repository
• make the generated OpenAPI entries resolve to separate same-origin assets

Why

Both remote specifications are named openapi.json. Without explicit directories, the generated llms.txt collapses them into /api-reference/openapi.json, which currently returns 404.

Closes #81.

Validation

• verified all 11 configured operations against the live Indexer and Gateway specifications
• pnpm exec cspell "**/*.{md,mdx,json,yml,yaml,ts,tsx,js,jsx}" --no-progress — 134 files, 0 issues
• mint dev — representative Indexer and Gateway API pages return 200 and render from their original remote sources
• mint validate — no new warning; retains the two existing snippet import warnings
• mint broken-links — unchanged from the existing 108-link / 48-file baseline