Add guidance to docs for upgrading multiple versions

[kbaley]

Occasionally we see users upgrading from very old major versions of our products. Usually, the first time we see this is when they run in to issues and raise a support case.

It might make sense to have docs pages for upgrading multiple major versions. i.e., to create a page that is titled "Upgrade from NServiceBus version 5 to version 7." We have pages for upgrading from version 5 to version 6, and for upgrading from version 6 to version 7, and we have guidance that recommends upgrading one major at a time. We are relying on other users doing research and putting these resources together into a coherent upgrade story.

We could consolidate all of that into one page with an SEO-friendly title for specific major versions. We can probably even automate the generation of these pages as we have a lot of the content and the metadata in docs already.

Depending on our cadence for releasing major versions, we may have more users running into these scenarios going forward.