docs: add upstream sync process documentation #369

Job	Run time
build / generate-license	49s
build / lint-python	8s
build / lint-toml	5s
build / lint-rust	14s
build / check-crates-patch	0s
build / ManyLinux arm64	8m 24s
build / ManyLinux x86_64	10m 15s
build / macOS arm64 & Windows (3.10, macos-latest)	8m 40s
build / macOS arm64 & Windows (3.10, windows-latest)	10m 19s
build / Source distribution	0s
build / build-macos-x86_64	0s
build / Build docs	1s
build / Merge build artifacts	1s
test	0s
	38m 56s

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