404 broken links

[lilin90]

Error Report

Please answer the following questions before submitting your issue. Thanks!

1. What is the URL/path of the document related to this issue?

   See this PR *: fix broken links #17041, which fixed 404 links for one branch. This PR has been cherry-picked to release-8.0 and release-7.5 branches, but 404 links also exist in other branches.

   Notes:

   • Different branches might have different 404 links.
   • Before you create a PR to a corresponding release-x branch, make sure the branch is active. You can check the active versions by clicking the version box on the top left corner of https://docs.pingcap.com/tidb/stable. Only update active branches because updating to inactive branches won't show on the docs.pingcap.com site.

2. How would you like to improve it?

   It would be better to fix all 404 links for all branches, but this can take a lot of time to check and fix them all. If you get some time and have interest, welcome to help create a PR or PRs to help fix them.

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Handling progress:

• Fix 404 broken links in release-7.6
• Fix 404 broken links in release-7.4
• Fix 404 broken links in release-7.1
• Fix 404 broken links in release-6.5
• Fix 404 broken links in release-6.1