Document how to use a persistent build directory and associated caveats

- Explain why you'd want to use the ``build-dir`` option
- Mention it works properly when using ``--cross-file`` (eg. https://github.com/scipy/scipy/blob/1c836efe5ff37ffa4490756269b060a464690e62/.github/workflows/wheels.yml#L180-L188)
- Explain how ``build-dir`` works
- Warn that ``meson-python`` version mismatches in the build directory can cause trouble