releases: add tidb 4.0.15 release notes (#6475)#6548

ti-chi-bot merged 15 commits into

pingcap:release-5.2pingcap/docs:release-5.2from

ti-chi-bot:cherry-pick-6475-to-release-5.2ti-chi-bot/docs:cherry-pick-6475-to-release-5.2Copy head branch name to clipboard