implement `pretty_help` #958

Job	Run time
Rustfmt	5s
Build docs	1m 16s
Test (MSRV)	1m 25s
Clippy	45s
Test (stable)	2m 0s
Test (beta)	2m 0s
Test (nightly)	2m 3s
Test (macOS)	1s
Test (Windows)	4m 50s
Test (no features)	1m 21s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 14s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 15s
	18m 15s

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