user feedback: Some small improvements to migration guides

[javabster]

I received some feedback that the migration guides sometime present a ton of options for people to choose from, but what is missing is some intuition to help a user decide which of the alternative options is best for them. For example in the simulator migration guide we show information for FakeBackend, AerSimulator, Clifford... etc, but we don't provide much guidance on which should a user choose to use.

Perhaps this could be solved by adding a short comparison table (or even just a paragraph in the intro) that compares the different tools, what features they have, what situations you might want to use them for etc.