bpo-45459: Add Py_buffer to limited API (GH-29991)

- [x] ``Py_buffer`` struct
- [x] ``PyBuffer_*()`` API functions
- [x] ``PyBUF_*`` constants
- [x] ``Py_bf_getbuffer`` and ``Py_bf_releasebuffer`` type slots
- [x] ``PyMemoryView_FromBuffer()`` API
- [x] tests for limited API
- [x] ``make regen-limited-abi``
- [x] documentation update
- [ ] export ``PyPickleBuffer*()`` API ???