docs: add Starlight documentation site

[mvantellingen]

What

Adds a comprehensive documentation site under docs/, built with Astro Starlight and deployed to GitHub Pages.

Live URL once merged + Pages enabled: https://labd.github.io/commercetools-node-mock/

Contents

• Getting started — introduction, installation, quick start (Vitest + msw), how it works
• Configuration — options reference, authentication
• Usage — seeding data (via the API), querying data, msw integration
• Storage backends — overview, in-memory, SQLite, custom
• Running as a server — standalone (runServer) + Docker
• Recipes — Vitest setup, cart flow, custom types, fishery fixtures
• Reference — API coverage & limitations, CommercetoolsMock, ProjectAPI, storage classes, errors
• Homepage with install tabs (pnpm/npm/yarn) and a "Built by Lab Digital and Evolve" section with both logos

Notes

• docs/ is a pnpm workspace package; the library's pnpm check is unaffected (docs/ is excluded in biome.json).
• SEO/GEO output baked in: llms.txt / llms-full.txt, sitemap, and full-text search.
• Content-based seeding is done through the API — unsafeAdd/getBaseResourceProperties are intentionally not documented (to be deprecated).

To go live after merge

1. Repo Settings → Pages → Source: "GitHub Actions" (one-time).
2. The docs workflow deploys automatically on pushes to main touching docs/**.

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Latest commit: 98f3f8c

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