Add Opflow Migration Guide

[ElePT]

What should we add?

To follow through with our plan to deprecate qiskit.opflow, we need a clear migration guide, and most likely a new documentation sub-section called migrations (@1ucian0).

I am currently working on this guide on a fork of the documentation repo, and will open a PR once the content is complete. In the meantime, you can track the progress of the current WIP document here.

[jakelishman]

While we're still deploying the Terra documentation via the metapackage, this might need a little bit of extra infrastructure on that side as well because there's an assumption there that all the docs we want to display are stored in docs/apidocs in Terra (our build process is suboptimal haha). It won't be too hard to do if we need to, though.

[woodsp-ibm]

Related to #9176