Doc: add discord badge #2809

Job	Run time
external-stores (stores/rocksstore-v2)	5m 55s
cluster-benchmark (cluster_benchmark)	47s
openraft-feature-test (nightly)	50s
openraft-feature-test (nightly, serde)	49s
openraft-test (stable, 0)	2m 52s
openraft-feature-test (nightly, single-term-leader)	50s
openraft-test (nightly, 30)	3m 9s
openraft-feature-test (nightly, single-term-leader,serde,singlethreaded)	50s
openraft-test (nightly, 0, single-term-leader)	2m 55s
openraft-test-bench (nightly)	56s
Build (nightly, bench,serde,bt,singlethreaded)	49s
tests-feature-test (nightly)	2m 41s
tests-feature-test (nightly, single-term-leader)	2m 49s
tests-feature-test (nightly, loosen-follower-log-revert)	2m 40s
examples (stable, raft-kv-memstore)	2m 25s
examples (stable, raft-kv-rocksdb)	8m 6s
examples (nightly, raft-kv-memstore)	2m 9s
examples (nightly, raft-kv-rocksdb)	12m 11s
lint	6m 32s
stores (memstore)	45s
stores (rocksstore)	6m 2s
stores (rocksstore-compat07)	6m 30s
stores (sledstore)	55s
	1h 14m 27s

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