Clarify documentation: three approaches, explicit terminology, STJ source generator

[Copilot]

The Overview section incorrectly listed JsonDateTimeFormatAttribute and DateTimeConverterResolver as separate approaches when they must be used together. Generic terms like "source generator" and "resolver" lacked precision. JsonDateTimeFormatConverter<T> was documented as source generator-only despite supporting reflection.

Changes

• Overview restructured from 4 to 3 approaches: Combined JsonDateTimeFormatAttribute + DateTimeConverterResolver as a single approach since the attribute is non-functional without the resolver
• Explicit terminology: "System.Text.Json source generator" replaces "source generator"; "DateTimeConverterResolver" replaces "resolver"
• JsonDateTimeFormatConverter clarified: Now documents support for both reflection-based and source generator scenarios; added "Does not require DateTimeConverterResolver" to pros
• Consistency: Updated all references throughout Prerequisites, Quick Start, Usage Scenarios, Available Components, and Quirks sections

Example

Before:

2. **`JsonDateTimeFormatAttribute`** - Clean attribute for source generators with .NET 9+ resolver (no warnings)
4. **`DateTimeConverterResolver`** - Contract customization for .NET 9+ source generators

After:

2. **`JsonDateTimeFormatAttribute` + `DateTimeConverterResolver`** - Clean attribute-based approach for System.Text.Json source generator with .NET 9+ (no warnings)

Original prompt

I think that overview that copilot created is a little bit off

## Overview

This package provides four ways to specify custom date formats for `DateTime`, `DateTimeOffset`, `DateOnly`, `TimeOnly`, and their nullable counterparts when serializing and deserializing JSON using `System.Text.Json`:

1. **`JsonDateTimeConverterAttribute`** - Simple attribute-based approach (reflection only, or .NET 9+ with resolver but produces warnings)
2. **`JsonDateTimeFormatAttribute`** - Clean attribute for source generators with .NET 9+ resolver (no warnings)
3. **`JsonDateTimeFormatConverter<T>`** - Type-safe converter for source generators (all .NET versions)
4. **`DateTimeConverterResolver`** - Contract customization for .NET 9+ source generators

That JsonDateTimeFormatAttribute can only work in conjuction when used with DateTimeConverterResolver
Mentioning the JsonDateTimeFormatAttribute and DateTimeConverterResolver seperatly is awkward, because the
JsonDateTimeFormatAttribute without DateTimeConverterResolver doesn't do anything.
"Simple attribute-based approach (reflection only, or .NET 9+ with resolver but produces warnings)" also is akward first of all instead of "resolved" should use
word DateTimeConverterResolver directly, also it's or with source generator net 9+ using the DateTimeConverterResolver just make the it all professional for the docs.
"4. DateTimeConverterResolver - Contract customization for .NET 9+ source generators" i think everywhere there is says source gen is better to specify that we are talking about STJ sourcegen https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/source-generation

Also in section "Source Generator with Format Converter (.NET 6+)" there is another Pros that
it doesn't require any DateTimeConverterResolver, it will just work if you use it with the normal JsonSerializerContext
it works both sourcegenerator and reflection based

so i guess here: 3. JsonDateTimeFormatConverter<T> - Type-safe converter for source generators (all .NET versions)
you could mention it all version and reflcetion and sourcegen

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.30%. Comparing base (192c762) to head (9f411a9).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

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  Coverage   92.30%   92.30%           
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  Files          13       13           
  Lines         156      156           
  Branches       35       35           
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  Hits          144      144           
  Misses          7        7           
  Partials        5        5           

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