book: add FROST Server section #1525

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Job	Run time
build with default features	27s
build with no_std (ristretto255)	25s
build with no_std (ed25519)	27s
build with MSRV (1.66.1)	2m 44s
actionlint	5s
build with no_std (p256)	27s
build with no_std (secp256k1)	27s
Clippy	1m 0s
build with no_std (secp256k1-tr)	29s
Check Rust doc	30s
Rustfmt	16s
build with no_std (rerandomized)	23s
Check if automatically generated code is up to date	28s
test on beta	2m 23s
	10m 31s

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