DOC: add sections about big new features (CoW, string dtype) to 3.0.0 whatsnew notes
We don't actually yet list the bigger features (string dtype, CoW, no silent downcasting) in the 3.0.0 whatsnew page, so starting to do that here.

Already pushed a section about string dtype, will further add a section about CoW and the downcasting.
This pull request is stale because it has been open for thirty days with no activity. Please [update](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#updating-your-pull-request) and respond to this comment if you're still interested in working on this. Going to merge this to have something in the docs, we can always further refine it so more feedback is certainly welcome!