[Docs] Update English version dataset_preparer.md

[Lum1104]

Motivation

The document of dataset_preparer from user guides only has the Chinese version.

Modification

I translate it into English version and put it into the en folder.

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects.
• CLA has been signed and all committers have signed the CLA in this PR.

[CLAassistant]

All committers have signed the CLA.

[OpenMMLab-Assistant-004]

Hi @Lum1104 ！We are grateful for your efforts in helping improve mmocr open-source project during your personal time.

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