DOC: add pandas 3.0 migration guide for the string dtype
This PR starts adding a migration guide with some typical issues one might run into regarding the new string dtype when upgrading to pandas 3.0 (or when enabling it in pandas 2.3).

(for now I just added it to the user guide, which is already a long list of pages, so we might need to think about better organizing this or putting it elsewhere)

Closes #59328
@simonjayhawkins thanks a lot for the proofreading!
 /preview Added three more sections based on the items listed in https://github.com/pandas-dev/pandas/issues/59328 cc @rhshadrach since you also already updated some code to work with the future string dtype, feel free to take a look and some feedback is certainly still welcome