Add Response Pagination Helper

[jordanpartridge]

Add a simple pagination helper for GitHub API responses:

$paginator = GitHub::repos()->paginate('owner/repo');
foreach ($paginator->items() as $repo) {
    // Process each repo
}

// Or get specific page
$page2 = GitHub::repos()->page(2)->get('owner/repo');

Implementation:

1. Create PaginatedResponse class
2. Add pagination methods to resources
3. Handle GitHub pagination headers
4. Add example to docs
5. Add tests

Makes working with large result sets easier.