Replace Document with EcoString #8660

Job	Run time
lint-build	52s
test-wasm	1m 32s
rustfmt	7s
test (stable, x86_64-unknown-linux-gnu)	3m 46s
validate	13s
test (stable, x86_64-unknown-linux-musl)	6m 43s
test (stable, aarch64-unknown-linux-gnu)	2m 8s
test (stable, aarch64-unknown-linux-musl)	2m 49s
test (stable, x86_64-apple-darwin)	12m 38s
test (stable, x86_64-pc-windows-msvc)	6m 52s
test (stable, aarch64-apple-darwin, macos-latest, arm64, cargo, true)	5m 23s
test-projects	1m 46s
	44m 49s

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