ci: start building docs for serenity-next #888

Job	Run time
Build docs	2m 20s
Clippy	1m 21s
Rustfmt	11s
Test (MSRV)	2m 2s
Test (stable)	4m 15s
Test (beta)	3m 10s
Test (nightly)	3m 25s
Test (macOS)	4m 55s
Test (Windows)	9m 2s
Test (no features)	2m 19s
Test (all features + simd_json, cargo test --all-features --features serenity/simd_json --lib --t...	2m 4s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	2m 35s
	37m 39s

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