DOC: [ArrowStringArray] release note and other documentation
extremely draft status. placeholder for discussion.

we may want to combine this with #39908, and merge #40708 before #39908

#39908 makes changes to the user facing api that affects the existing StringDtype so that may need an separate section in the release notes.

other sections of the documentation may also need updating. e.g. doc/source/user_guide/text.rst, doc/source/development/roadmap.rst and doc/source/user_guide/basics.rst