*: move external storage to dumpling and lightning docs

[shichun-0415]

What is changed, added or deleted? (Required)

After BR refactor in pingcap/docs-cn#11316, external storage is replaced by Backup Storages. However, because some description about Dumpling and Lightning are still be useful and referenced by Dumpling and Lightning docs, external storage is removed from the navigation but kept in the docs repo. It would be better to move information about Dumpling and Lightning to the docs of these two tools, instead of being wrapped in br docs.

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

• This PR is translated from: *: move external storage to dumpling and lightning docs docs-cn#12888
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Do your changes match any of the following descriptions?

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

[ti-chi-bot]

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[Oreoxmt]

Rest LGTM

[shichun-0415]

/merge

[ti-chi-bot]

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

Commit hash: 8fad0d7

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.4: #12361.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #12362.