Update Availability article

Add availability article

re-ordering articles

update idempotency article

Add push vs pull architecture article

Add stateful vs stateless design article

Update tradeoff section with more articles

Remove duplicate link for "top 15 tradeoffs"

Add tradeoff section

Update strong vs eventual consistency article

Update Rate Limiting article

Add rate limiting code

Update data redundancy article

Update CDN Article

Reorder sections