releases: add TiDB 5.1.1 release notes (#6030)#6065

Merged

ti-chi-bot merged 2 commits intopingcap:release-5.1from ti-chi-bot:cherry-pick-6030-to-release-5.1

Jul 30, 2021

+160-2

Commits on Jul 30, 2021

This is an automated cherry-pick of pingcap#6030

zhouqiang-cl
authored and
ti-chi-bot
committed
Update TOC.md
TomShawn
authored