Update upgrade docs for 6.0.0 about stack-docs HOT 3 CLOSED

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.

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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

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

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