Upgrade overview page should act as a map to child pages

[shainaraskas]

https://www.elastic.co/docs/deploy-manage/upgrade

Upgrades are a complex process and the paths each user needs to follow depends on their deployment type + context. We should enhance this page to provide a map of the upgrade process and the pages that the users will need to touch (e.g. when should I look at the "plan your upgrade' page"?)

related internal slack discussion

[shainaraskas]

restore note about out-of-order releases: https://www.elastic.co/guide/en/elasticsearch/reference/8.18/setup-upgrade.html#upgrade-newer-releases

[eedugon]

Attemtping to solve this with #2296