DOC: add section about upcoming pandas 3.0 changes (string dtype, CoW) to 2.3 whatsnew notes
This is largely copied from the equivalent notes in the 2.2 release notes at https://pandas.pydata.org/pandas-docs/stable/whatsnew/v2.2.0.html#upcoming-changes-in-pandas-3-0, with some updates (and some new content copied from WIP 3.0 release notes in https://github.com/pandas-dev/pandas/pull/61724)
/preview LGTM @jorisvandenbossche is the "TODO add link to migration guide" going to be done here or a follow up? There is a mix of `code-block:: python` and `code-block:: ipython`. Intentional? > LGTM @jorisvandenbossche is the "TODO add link to migration guide" going to be done here or a follow up?

Planning to merge that migration guide now first and then update here with a link > There is a mix of `code-block:: python` and `code-block:: ipython`. Intentional?

Fixed (although it was for code blocks where no code prompt was included, and at that point there is not really a difference, I think) /preview Website preview of this PR available at: https://pandas.pydata.org/preview/pandas-dev/pandas/61795/