[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.

Start with the linked high-level Elastic Stack upgrade overview and the upgrade discussion referenced in the issue. Define the safest practices for larger deployments and product-specific guidance, then verify the documentation covers upgrade order and links each product's guidance.

Written by the indexing model from the issue text.