Fix MD book generation #4037

Job	Run time
Book tests	3m 36s
Bitrot check	1m 34s
Build target wasm32-unknown-unknown	4m 0s
Code coverage	21m 18s
Build target wasm32-wasi	3m 51s
Intra-doc links	3m 43s
Rustfmt	18s
Test on i686-unknown-linux-gnu	15m 55s
Test on ubuntu-latest	9m 27s
Test on i686-unknown-linux-gnu with beta features	15m 53s
Test on windows-latest	12m 15s
Test on i686-unknown-linux-gnu with nightly features	17m 34s
Test on macOS-latest	14m 13s
Test on ubuntu-latest with beta features	9m 32s
Test on windows-latest with beta features	12m 26s
Test on macOS-latest with beta features	11m 18s
Test on ubuntu-latest with nightly features	10m 46s
Test on windows-latest with nightly features	13m 34s
Test on macOS-latest with nightly features	13m 1s
	3h 14m 14s

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