docs: correct global converter write examples

[skytin1004]

Purpose of the pull request

The global XLSX/XLS write examples register a converter with a STRING key, but ordinary Excel writes look it up with a null Excel type key. As a result, the custom transformation is skipped. This PR corrects the examples to match that lookup.

Related: #889

What's changed?

• Use a null Excel type key for global XLSX/XLS writing in the English and Chinese examples; retain STRING for CSV writing and reading string cells.
• Clarify that registered converters must match the lookup key, and the returned WriteCellData determines the output cell type.
• Add XLSX/XLS tests that check the written values and field-level converter precedence.

Validation: both XLSX/XLS regression cases fail with the original registration and pass with the corrected key. All 9 CustomConverterTest cases pass on JDK 11, and the fork PR's Java and documentation CI checks pass.

Checklist

• I have read the Contributor Guide.
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.

The Chinese documentation updates were translated with AI. A review of the Chinese wording would be appreciated.