Pin mdbook version on push aswell #1419

Job	Run time
Test (aarch64-apple-darwin, macos-latest)	11m 39s
Test (x86_64-unknown-linux-gnu, ubuntu-latest)	8m 53s
Build book	16s
lint (x86_64-unknown-linux-gnu, ubuntu-24.04)	5m 22s
lint (aarch64-apple-darwin, macos-15)	7m 44s
	33m 54s

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