docs: expand Sourcey entry with tested usage

[offer-catalog-worker]

Summary

• Expand the existing Sourcey entry with an overview of its OpenAPI documentation output.
• Add a tested installation and minimal build example.
• Describe reproducible CI usage and the current pnpm packaging caveat.
• Keep the existing docs and MCP categories while adding the Node.js runtime tag.

Validation

• pnpm test: 13 tests passed.
• pnpm lint: completed with two pre-existing warnings and no errors.
• astro check: 0 errors, 0 warnings, 0 hints.
• The production build rendered /tools/sourcey/; a local HTTP request returned 200 and found the body link to https://sourcey.com/.
• Sourcey 3.6.5 built the included minimal OpenAPI Description into nine static files, also verified over local HTTP.

The Netlify adapter's final packaging hook could not create a symlink under this Windows environment (EPERM) after all pages had rendered. The Sourcey page itself was generated and inspected successfully.

[netlify]

👷 Deploy request for openapi-dot-tools pending review.

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