feat: add tanstack start example and add readme.md file to astro example

[shomriddho]

This PR adds a new example application demonstrating how to integrate Payload CMS with TanStack Start and TanStack Query for modern full-stack development.

What?

• Adds examples/tanstack-start/ directory with complete example application
• Includes Payload CMS configuration with Posts collection and CRUD operations
• TanStack Start frontend with TanStack Query for efficient data management
• Demonstrates server-side data fetching and client-side mutations
• Shows proper monorepo setup with workspace dependencies

Why?

• Provides developers with a reference implementation for TanStack Start + Payload CMS integration
• Shows best practices for data fetching and state management with TanStack Query
• Demonstrates modern React patterns with server/client components
• Expands the examples directory with cutting-edge technology stack
• Complements the existing examples/astro by showing integration with different frontend frameworks

How?

This example is adapted from the existing examples/astro but uses TanStack Start instead of Astro:

• Monorepo configuration with pnpm workspaces (same as astro)
• Server-side data fetching in TanStack Router loaders (vs Astro server actions)
• Client-side mutations with automatic cache invalidation (vs Astro actions)
• CORS configuration for cross-origin API calls
• Public access controls for demo purposes (same as astro)
• Proper TypeScript integration throughout

Features Demonstrated

• ✅ Payload CMS Posts collection with title field
• ✅ Sample data initialization on startup
• ✅ TanStack Query for data fetching and caching
• ✅ Create, read, update, delete operations
• ✅ Real-time UI updates without page reloads
• ✅ Modern React Server Components architecture
• ✅ Full TypeScript support

Comparison to examples/astro

While examples/astro shows Payload integration with Astro framework, this example demonstrates:

• TanStack Start as an alternative modern React framework
• TanStack Query for advanced data management
• Different architectural patterns for server/client data flow
• Enhanced developer experience with real-time updates

Fixes # (no specific issue)

[shomriddho]

Hey @denolfe @AlessioGr 👋
This PR adds a new TanStack Start example alongside the existing Astro one.

Main things to review:
• Overall structure of the new examples/tanstack-start example
• Whether this fits well alongside examples/astro from a docs/examples POV

I’ve resolved the merge conflicts and all checks should be green now.
No rush at all — appreciate any feedback when you have time 🙏

[shomriddho]

Hey @denolfe @AlessioGr 👋 This PR adds a new TanStack Start example alongside the existing Astro one.

Main things to review: • Overall structure of the new examples/tanstack-start example • Whether this fits well alongside examples/astro from a docs/examples POV

I’ve resolved the merge conflicts and all checks should be green now. No rush at all — appreciate any feedback when you have time 🙏

[shomriddho]

Hey @denolfe @AlessioGr 👋 This PR adds a new TanStack Start example alongside the existing Astro one.

Main things to review: • Overall structure of the new examples/tanstack-start example • Whether this fits well alongside examples/astro from a docs/examples POV

I’ve resolved the merge conflicts and all checks should be green now. No rush at all — appreciate any feedback when you have time 🙏