Add caching and pagination guidelines for sitemaps

[flamingotiger]

Notes:

• Google limits sitemaps to 50,000 URLs and 50MB per file, so PAGE_SIZE should stay well below this.
• Ensure proper DB indexes (e.g., published, updatedAt) to avoid expensive sort operations.
• Prefer Next.js built-in sitemap API when possible; route handlers are useful when custom caching logic is required.