Docs: Update CsvProvider and JsonProvider for DateOnly/TimeOnly inference

[Copilot]

The CSV and JSON provider documentation did not reflect the DateOnly/TimeOnly inference support added in #1609.

CsvProvider.fsx

• MSFT.csv example: Updated Date column type description from DateTime → DateOnly (on .NET 6+)
• Inference section: Added paragraph noting that date-only strings auto-infer as DateOnly on .NET 6+; TimeOnly is never auto-inferred (ambiguous with TimeSpan) and requires explicit annotation
• Nullable types: Updated to include DateOnly (nullable) and DateTimeOffset (optional)
• Valid type list: Added dateonly, dateonly?, dateonly option, timeonly, timeonly?, timeonly option (.NET 6+ only)
• New subsection: "DateOnly and TimeOnly (on .NET 6+)" with a schema example:

EventDate,Duration (timeonly?)
2023-06-01,08:30:00
2023-06-02,

EventDate is auto-inferred as DateOnly; Duration requires the explicit timeonly? annotation.

JsonProvider.fsx

• Added "Inferring date types" subsection: date-only strings (e.g. "2023-01-15") infer as DateOnly on .NET 6+; full datetime strings remain DateTime.

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