Doc: add faq: "What will happen when data gets lost" #2715

Job	Run time
openraft-test-bench (nightly)	53s
cluster-benchmark (cluster_benchmark)	1m 24s
external-stores (stores/rocksstore-v2)	11m 16s
openraft-feature-test (nightly)	48s
openraft-feature-test (nightly, serde)	49s
openraft-test (stable, 0)	4m 22s
openraft-feature-test (nightly, single-term-leader)	1m 8s
openraft-test (nightly, 30)	5m 35s
openraft-feature-test (nightly, single-term-leader,serde,singlethreaded)	1m 9s
openraft-test (nightly, 0, single-term-leader)	2m 58s
stores (memstore)	1m 19s
Build (nightly, bench,serde,bt,singlethreaded)	47s
stores (rocksstore)	12m 28s
stores (rocksstore-compat07)	5m 59s
stores (sledstore)	50s
examples (stable, raft-kv-memstore)	2m 13s
examples (stable, raft-kv-rocksdb)	15m 4s
lint	6m 21s
examples (nightly, raft-kv-memstore)	2m 9s
examples (nightly, raft-kv-rocksdb)	12m 28s
tests-feature-test (nightly)	2m 40s
tests-feature-test (nightly, single-term-leader)	2m 39s
tests-feature-test (nightly, loosen-follower-log-revert)	2m 39s
	1h 37m 58s

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