Doc: faq: add differences from std raft #2851

Job	Run time
cluster-benchmark (cluster_benchmark)	45s
lint	6m 24s
examples (stable, raft-kv-memstore)	2m 19s
external-stores (stores/rocksstore-v2)	5m 48s
examples (stable, raft-kv-rocksdb)	7m 54s
openraft-feature-test (nightly)	48s
examples (nightly, raft-kv-memstore)	2m 7s
openraft-test (stable, 0)	2m 57s
openraft-feature-test (nightly, serde)	59s
openraft-test-bench (nightly)	1m 0s
examples (nightly, raft-kv-rocksdb)	11m 50s
openraft-test (nightly, 30)	3m 18s
openraft-feature-test (nightly, single-term-leader)	48s
openraft-test (nightly, 0, single-term-leader)	3m 5s
openraft-feature-test (nightly, single-term-leader,serde,singlethreaded)	49s
stores (memstore)	52s
Build (nightly, bench,serde,bt,singlethreaded)	52s
stores (rocksstore)	5m 58s
tests-feature-test (nightly)	2m 47s
stores (rocksstore-compat07)	6m 5s
tests-feature-test (nightly, single-term-leader)	2m 39s
stores (sledstore)	55s
tests-feature-test (nightly, loosen-follower-log-revert)	2m 43s
	1h 13m 42s

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