fix: add .nojekyll file and verify book output during site build process #19

Job	Run time
runtime smoke	17s
backend features (default)	21s
parse conformance	20s
fmt + clippy	14s
backend features (llvm-backend)	1m 18s
type + lowering conformance	22s
SIMD contract (Cranelift)	18s
	3m 10s

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