Update upgrade docs for 6.0.0

[dedemorton]

Let's use this issue to organize the effort to update the upgrade docs for 6.0.0.

The first step is for everyone to take a look at the existing upgrade docs and identify what's missing or needs to be updated:

https://www.elastic.co/guide/en/elastic-stack/current/index.html

Last time around, we decided to point to the docs for each component for detailed instructions. Now is the time to decide if we want to continue with that approach.

• Update stack docs
  • Restructure content - break it up into smaller sections.
  • How to do a rolling upgrade from 5.x to 6.x
• Elasticsearch & X-Pack install and upgrades
• Kibana & X-Pack install and upgrades
• Logstash & X-Pack install and upgrades
• Beats install and upgrades

[nrichers]

Last time around, we decided to point to the docs for each component for detailed instructions. Now is the time to decide if we want to continue with that approach.

For Cloud and ECE, that's likely the best approach, especially since we're not directly tied to the Elastic Stack release cycle.

[clintongormley]

Last time around, we decided to point to the docs for each component for detailed instructions. Now is the time to decide if we want to continue with that approach.

I think this still makes sense, but we should have a separate section in the stack docs about how to do a rolling upgrade from 5.x to 6.x.

i'd also like to break up the pages in the stack docs already - each page is quite long and complicated. Could also do with some more headers to break up the content

[michaelbaamonde]

I wrote up #15, which is a superset of this issue. Thanks for getting the ball rolling here.