Improve docs: UpgradeScripts

[tunger]

The quickstart / documentation doesn't state that one needs to run some queries on the target database in order to use Nevermore.

https://github.com/OctopusDeploy/Nevermore/tree/master/source/Nevermore/UpgradeScripts

I figure it works best when using DbUp.
It would be good to mention this in the docs.

[PaulStovell]

You're right @tunger. Which solution do you prefer?

1. Explain that the scripts are needed (docs)
2. Find a way to not need the scripts (e.g., run it only if the feature is used, but do it automatically?)

With your support for other ID types the upgrade scripts might not even be needed, so I am leaning towards option 2.

[khalidabuhakmeh]

I would prefer 2. as it is more discoverable for folks getting started with Nevermore.

[akirayamamoto]

I added a section to the docs explaining that one should run the upgrade scripts while we don't have a way to automatically run those scripts.