Document pyo3-build-config "config files"

[davidhewitt]

In #1758 I added a PYO3_CONFIG_FILE environment variable which can be used to provide custom configuration for PyO3's build script.

I wanted to let this be "undocumented" in 0.14.2 so that we can have some time to learn about it before making it a stable documented feature. For 0.15, it would be nice to commit to a design and document it, as I think it can be useful for users (especially when cross-compiling).

(See #1758 (comment) for previous discussion.)

[davidhewitt]

(While doing this, it might be a good idea to update Architecture.md to explain the purpose pyo3-build-config)