sql: make global indexes a new doc and add more contents (#22017)#22146

ti-chi-bot[bot] merged 2 commits intopingcap:release-8.5pingcap/docs:release-8.5from

ti-chi-bot:cherry-pick-22017-to-release-8.5ti-chi-bot/docs:cherry-pick-22017-to-release-8.5Copy head branch name to clipboard