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

[sary8]

I've opened two PRs for this issue:

• *: fix 404 links in the v7.1 documentation #23898 for release-7.1
• *: fix 404 links in the v6.5 documentation #23899 for release-6.5

Notes on the scope I picked, in case it saves someone else duplicated work:

• Of the branches in the checklist above, I only covered release-7.1 and release-6.5. The PR template currently lists master, v9.0, v8.5, v8.1, v7.5, v7.1, and v6.5 as the target versions, so release-7.6, release-7.4, and release-6.1 look inactive and I skipped them per the note in the issue description.
• I also checked release-7.5, release-8.1, and release-8.5. None of them still carry the links that *: fix broken links #17041 fixed, so nothing seems to be left there.
• Besides the links from *: fix broken links #17041, both branches also had a 404 link to https://github.com/pingcap/tidb/blob/master/br/pkg/restore/systable_restore.go#L31 in br/br-snapshot-guide.md and faq/backup-and-restore-faq.md. The PRs fix that too.

One decision I'd like a maintainer's opinion on: for the pingcap/tidb source links, I pinned them to the matching release-x.y branch rather than to master as #17041 did. In master those paths have moved again (br/pkg/lightning to lightning, and br/pkg/restore/systable_restore.go to br/pkg/restore/snap_client), while both still exist in the release-7.1 and release-6.5 branches of pingcap/tidb, and release-7.5 already uses the version-pinned form. I'm happy to switch to the master form if you'd prefer consistency with #17041.

One more thing I ran into, which I did not include in these PRs: https://ask.pingcap.com/ no longer accepts connections and https://support.pingcap.com/ no longer resolves at all, but they are still linked from 74 and 79 files respectively in release-6.5 (similar numbers in release-7.1). master has replaced those "Need help?" blocks with Discord/Slack and https://tidb.support.pingcap.com/. That felt too large and too wording-dependent to fold into a link-fix PR. Would you like me to open a separate issue for it?