docs: document FFT table function

[DaZuiZui]

Summary

• Document the built-in FFT table function in the English and Chinese table-model SQL manuals.
• Add syntax, parameters, output columns, validation rules, examples, and known limitations.
• Link the FFT table-function section from the UDF/user manual pages.
• Keep the Master/Table English page synchronized with latest-Table.

Notes

The documentation describes zero-padding/truncation, the 65,536 transform-length limit, strict timestamp ordering, non-NULL numeric inputs, and approximate frequency results for irregular sampling intervals.

Related to apache/iotdb#18006

[DaZuiZui]

PR link：apache/iotdb#18006 (comment)