The "Standard date and time format strings" documentation page previously only covered DateTime and DateTimeOffset types, but did not include the DateOnly and TimeOnly types that were introduced in .NET 6. This update adds comprehensive coverage for these new types.

Changes Made

• Updated introduction: Added DateOnly and TimeOnly to the main description alongside DateTime and DateTimeOffset
• Added Notes section: New dedicated section explaining which format specifiers work with each type
• Updated format specifier table: Added examples showing DateOnly and TimeOnly usage for supported formats
• Enhanced parsing documentation: Added references to DateOnly.ParseExact and TimeOnly.ParseExact methods
• Updated round-trip format section: Explained how the "O"/"o" format specifier works with the new types

Format Specifier Compatibility

The documentation now clearly explains that:

• DateOnly supports date-related format specifiers: "d", "D", "M"/"m", "Y"/"y", "O"/"o", "R"/"r"
• TimeOnly supports time-related format specifiers: "t", "T", "O"/"o", "R"/"r"
• Both types throw FormatException for combined date/time formats: "f", "F", "g", "G", "s", "u", "U"

Examples Added

// DateOnly formatting
var date = new DateOnly(2009, 6, 15);
Console.WriteLine(date.ToString("d")); // 6/15/2009 (en-US)
Console.WriteLine(date.ToString("O")); // 2009-06-15

// TimeOnly formatting  
var time = new TimeOnly(13, 45, 30);
Console.WriteLine(time.ToString("T")); // 1:45:30 PM (en-US)
Console.WriteLine(time.ToString("O")); // 13:45:30.0000000

This ensures developers have complete documentation for all four .NET date/time types in one comprehensive reference.

Fixes #43423.

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