*: generalize and link to the external storage docs from Lightning (#5286)

[ti-srebot]

cherry-pick #5286 to release-4.0
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# In docs repo:
git pr https://github.com/pingcap/docs/pull/5315

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---

What is changed, added or deleted? (Required)

Fix #4445.

Generalize the "BR storage" page to "external storage", and talk about all associated migration services.

Link back to this page from their documents.

Follow-up: Change the file name for 外部存储 and remove it out of /br. @IANTHEREAL will work on its new position in TOC.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v5.0 (TiDB 5.0 versions)
• v4.0 (TiDB 4.0 versions) Have version-specific changes
• v3.1 (TiDB 3.1 versions)
• v3.0 (TiDB 3.0 versions)
• v2.1 (TiDB 2.1 versions)

What is the related PR or file link(s)?

• This PR is translated from: *: generalize and link to the external storage docs from Lightning docs-cn#5528 FIx typos in br external storages docs-cn#6018
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[ti-srebot]

@TomShawn you're already a collaborator in bot's repo.

[TomShawn]

/remove-label requires-followup

[TomShawn]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• TomShawn

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[TomShawn]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 5e9cb92

[TomShawn]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: cd9e466