ci: start building docs for serenity-next (#213) #889

Job	Run time
Clippy	1m 21s
Build docs	3m 33s
Rustfmt	9s
Test (MSRV)	2m 8s
Test (stable)	3m 15s
Test (beta)	3m 24s
Test (nightly)	3m 10s
Test (macOS)	4m 34s
Test (Windows)	9m 3s
Test (no features)	1m 55s
Test (all features + simd_json, cargo test --all-features --features serenity/simd_json --lib --t...	2m 31s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	2m 2s
	37m 5s

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