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Polish upstream-based bilingual README and add release downloads #4

Job	Run time
contract	4m 2s
test-linux-tooling	2m 58s
test-linux-rust (workspace-crates, -p webcodex-admin -p webcodex-runner-config -p webcodex-core -...	2m 34s
test-linux-rust (server, -p webcodex)	8m 52s
test-linux-rust (runner, -p webcodex-runner)	2m 3s
test-macos	0s
test-windows-core	0s
test-windows-runner	0s
test-windows-package	0s
test-linux-arm64	0s
test-windows-arm64	0s
test-windows	0s
test	5s
	20m 34s