Doc: add change log for 0.8.7 #2797

Job	Run time
cluster-benchmark (cluster_benchmark)	45s
Build (nightly, bench,serde,bt)	48s
openraft-feature-test (nightly)	47s
external-stores (stores/rocksstore-v2)	5m 50s
openraft-test-bench (nightly)	57s
openraft-feature-test (nightly, serde)	48s
openraft-test (stable, 0)	2m 57s
stores (memstore)	42s
openraft-feature-test (nightly, single-term-leader)	46s
openraft-test (nightly, 30)	3m 9s
stores (rocksstore)	6m 13s
openraft-test (nightly, 0, single-term-leader)	2m 52s
stores (rocksstore-compat07)	6m 22s
openraft-feature-test (nightly, single-term-leader,serde)	50s
stores (sledstore)	1m 24s
examples (stable, raft-kv-memstore)	2m 25s
examples (stable, raft-kv-rocksdb)	18s
examples (nightly, raft-kv-memstore)	2m 6s
examples (nightly, raft-kv-rocksdb)	18s
lint	6m 36s
	46m 53s

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