docs: add comprehensive documentation for useSWRInfinite revalidation behavior and infinite loading example

[ruguoba]

Description

This PR adds comprehensive documentation for useSWRInfinite's revalidation behavior and a practical infinite loading example, addressing the confusion raised in #785 and #883.

Changes

1. Revalidation Behavior Documentation (docs/infinite-revalidation.md)

• Added detailed explanation of why only the first page is revalidated by default
• Documented all relevant configuration options:
  • revalidateFirstPage
  • revalidateAll
  • revalidateIfStale
  • Standard SWR options
• Provided practical use cases and code examples
• Added best practices section
• Included FAQ section for common questions

2. Infinite Loading Example (examples/infinite-loading-intersection-observer.md)

• Complete example using useSWRInfinite with IntersectionObserver
• Demonstrates how to use useCallback to prevent infinite loops
• Includes both JavaScript and TypeScript versions
• Shows alternative approach with click button
• Covers cleanup and memory leak prevention

Why This Documentation Is Needed

Issue #785 - Revalidation Behavior Confusion

Many users have been confused by the default behavior of useSWRInfinite where only the first page is revalidated when loading new pages. This documentation clarifies:

1. Performance optimization: Why revalidating all pages would be a performance disaster
2. Timeline design: Why first-page revalidation makes sense for most infinite loading use cases
3. Configuration options: How to customize the behavior for different use cases

Issue #883 - Infinite Loop with IntersectionObserver

Users reported infinite loops when using setSize with IntersectionObserver. The new example demonstrates:

1. Proper memoization: Using useCallback to prevent function recreation
2. Observer management: Using useRef to store the observer
3. Cleanup: Proper cleanup to prevent memory leaks
4. TypeScript support: Full TypeScript example included

Related Issues

• Closes Better documentation on the revalidation behavior of the infinite hook #785
• Addresses add useCallback in useSWRInfinite example when need to use setSize #883

Testing

This is a documentation-only change, so no code testing is required. The documentation has been reviewed for accuracy based on the existing codebase and the discussion in #785 and #883.

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Note: This documentation aims to help users understand the design decisions behind useSWRInfinite's revalidation behavior and provide clear guidance on how to implement infinite loading with IntersectionObserver without encountering infinite loops.