[Docs] Provide best practices for the safest way of upgrading the Elastic Stack

Last week we discussed our upgrade documentation and ways we can improve the upgrade experience; for larger deployments, the proper order becomes important.

While we provide [this high level overview](https://www.elastic.co/guide/en/elastic-stack/current/upgrading-elastic-stack.html#upgrade-order-elastic-stack) for the order of the products, we should also include best practices around the safest way to upgrade the stack, as well as specific guidance for each product.

Regarding product specific guidance, I will link those to this issue once they are opened.