Guide for upgrades and backouts

[1000TurquoisePogs]

This PR acts upon zowe/community#2305

The notable changes are:

1. Added reference for the Unix content associated with a Zowe install, to be able to refer to it on the backout procedure
2. Duplicated the content of the "migrate from vX to v3" guide to put within the upgrade guide. It was a really good document but it's in a v3 specific place so I want to put that content in a good place for the long run and maintain it. Because that document has been so good, I duplicated the content in order to not disrupt it, for people who have bookmarked it or something.

[github-actions]

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[github-actions]

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[github-actions]

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