book: use `mkdocs` instead of `mdbook` #950

Job	Run time
Build no_std (nightly)	1m 43s
Format	7s
Build WASM (stable, -p nostr)	17s
Build (stable, -p nostr)	1m 16s
Build WASM (stable, -p nostr-sdk)	34s
Build WASM (stable, -p nostr-js)	35s
Build (stable, -p nostr --no-default-features --features alloc)	1m 31s
Build WASM (1.64.0, -p nostr)	4m 35s
Build (stable, -p nostr --no-default-features --features "alloc all-nips")	34s
Build (stable, -p nostr --features blocking)	1m 53s
Build WASM (1.64.0, -p nostr-sdk)	5m 36s
Build (stable, -p nostr-sdk)	2m 36s
Build WASM (1.64.0, -p nostr-js)	4m 30s
Build (stable, -p nostr-sdk --no-default-features)	1m 57s
Build (stable, -p nostr-sdk --features blocking)	2m 1s
Build (stable, -p nostr-ffi)	2m 3s
Build (stable, -p nostr-sdk-ffi)	2m 30s
Build (1.64.0, -p nostr)	6m 51s
Build (1.64.0, -p nostr --no-default-features --features alloc)	4m 42s
Build (1.64.0, -p nostr --no-default-features --features "alloc all-nips")	6m 30s
Build (1.64.0, -p nostr --features blocking)	7m 50s
Build (1.64.0, -p nostr-sdk)	6m 54s
Build (1.64.0, -p nostr-sdk --no-default-features)	5m 35s
Build (1.64.0, -p nostr-sdk --features blocking)	6m 28s
Build (1.64.0, -p nostr-ffi)	5m 53s
Build (1.64.0, -p nostr-sdk-ffi)	6m 16s
	1h 31m 17s

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