book: use `mkdocs` instead of `mdbook` #961

Job	Run time
Format	4s
Build WASM (stable, -p nostr)	29s
Build no_std (nightly)	1m 59s
Build WASM (stable, -p nostr-sdk)	45s
Build (stable, -p nostr)	1m 22s
Build WASM (stable, -p nostr-js)	25s
Build (stable, -p nostr --no-default-features --features alloc)	1m 3s
Build WASM (1.64.0, -p nostr)	4m 29s
Build (stable, -p nostr --no-default-features --features "alloc all-nips")	41s
Build WASM (1.64.0, -p nostr-sdk)	5m 50s
Build (stable, -p nostr --features blocking)	1m 50s
Build WASM (1.64.0, -p nostr-js)	4m 54s
Build (stable, -p nostr-sdk)	3m 3s
Build (stable, -p nostr-sdk --no-default-features)	1m 23s
Build (stable, -p nostr-sdk --features blocking)	2m 20s
Build (stable, -p nostr-ffi)	2m 1s
Build (stable, -p nostr-sdk-ffi)	3m 19s
Build (1.64.0, -p nostr)	5m 56s
Build (1.64.0, -p nostr --no-default-features --features alloc)	6m 14s
Build (1.64.0, -p nostr --no-default-features --features "alloc all-nips")	5m 31s
Build (1.64.0, -p nostr --features blocking)	6m 2s
Build (1.64.0, -p nostr-sdk)	6m 42s
Build (1.64.0, -p nostr-sdk --no-default-features)	6m 28s
Build (1.64.0, -p nostr-sdk --features blocking)	6m 44s
Build (1.64.0, -p nostr-ffi)	7m 51s
Build (1.64.0, -p nostr-sdk-ffi)	7m 44s
	1h 35m 9s

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