Use a permanent build directory

Todo

• 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 user-configured build directories are “use at your own risk”

• Warn that meson-python version mismatches in the build directory can cause trouble