Backports upgrade guide to 7.17

[natasha-moore-elastic]

Contributes to #5302.

Preview: Upgrade from 7.17 to an 8.x version

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[natasha-moore-elastic]

@jmikell821 The build is failing because upgrading-elasticsearch.html and upgrading-kibana.html don't exist in 7.17 Elastic Stack docs, so {stack-ref}/upgrading-elasticsearch.html and {stack-ref}/upgrading-kibana.html on lines 67 and 76 don't work. The pages exist in 8.0 Stack docs, but according to https://github.com/elastic/docs/blob/master/shared/attributes.asciidoc, we don't have an attribute for .../elastic-stack/8.0. It seems like the only option would be to use an absolute URL?