Code examples for perf-oriented API suggest inefficient code

### Type of issue

Other (describe below)

### Description

The first code example under `Utf8Parser and Utf8Formatter` contains this code in the overridden `Write` method:

```
Span utf8Date = new byte[29];
```

Allocating a new byte array every time a `DateTime` value is written seems to be a considerable overhead. Allocating byte array on the stack instead would be more efficient.

Also, that section begins with the statement "if your input DateTime or DateTimeOffset text representations are compliant with one of the "R", "l", "O", or "G" standard date and time format strings". It seems like this is not a limitation and the value can be serialized using any supported format. For example, here's how DateTime value can be written in "s" format:

```
public override void Write(Utf8JsonWriter writer, DateTimeOffset value, JsonSerializerOptions options)
{
Span span = stackalloc char[32];
value.TryFormat(span, out var count, "s");
writer.WriteStringValue(span[..count]);
}
```

### Page URL

https://learn.microsoft.com/en-us/dotnet/standard/datetime/system-text-json-support#-and-

### Content source URL

https://github.com/dotnet/docs/blob/main/docs/standard/datetime/system-text-json-support.md

### Document Version Independent Id

77358796-3ee2-9b86-7e0a-92c60ebc371a

### Platform Id

45e1d7c2-9630-1091-5921-5194b2bfef72

### Article author

@layomia

### Metadata

* ID: eb41b02f-8fe9-44a1-544f-b6016f1ff4b3
* PlatformId: 45e1d7c2-9630-1091-5921-5194b2bfef72
* Service: **dotnet-fundamentals**

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