dumpling: merge userguide into docs

[dveeden]

What is changed, added or deleted? (Required)

Merge the info from https://github.com/pingcap/tidb/blob/master/dumpling/docs/en/user-guide.md into pingcap/docs.

Note: there is also https://github.com/pingcap/tidb/blob/master/dumpling/docs/cn/user-guide.md that can help for translation

See also: pingcap/tidb#58679

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For details, see tips for choosing the affected versions.

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