docs(fast-check): add package README and integrations docs

[radhityaz]

Summary

• Add a package README for @ark/fast-check with install steps, arkToArbitrary usage, supported schema categories, and unsupported-schema behavior.
• Document the integration on arktype.io under Integrations, including sidebar navigation.
• Update llms.txt so the new integrations content is included in exported docs.

Closes #1243

Test plan

• pnpm exec prettier --check on edited formattable files
• pnpm --filter @ark/docs build

Made with Cursor

[pullfrog]

✅ No new issues found.

Reviewed changes — adds package README for @ark/fast-check and integrations documentation on arktype.io.

• ark/fast-check/README.md — new README covering install, arkToArbitrary usage, and supported schema categories.
• ark/docs/content/docs/integrations/index.mdx — new fast-check section with code example and link to the package README.
• ark/docs/content/docs/integrations/meta.json — sidebar entry for fast-check.
• ark/docs/public/llms.txt — mirrored integrations content for LLM export.

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