[QDC Feedback] Add marginal counts info to migration guide

[javabster]

We got some feedback from office hours that the docs migration guide does not help guide how to easily get a list with marginal counts. I'm not sure which migration guide specifically they are referring to though.

[beckykd]

From Jessie:

Looks like the old Result class had a marginal_counts convenience method (backend.run returns Result) that the new sampler result class doesn't

So we probably need to document how to do that with the new sampler result.