No instructions for registering JsonConverterFactory implementations

### Type of issue

Missing information

### Description

The documentation elaborates very well how to register a custom converter, however, it shows how and when to create a converter _factory_ but not how to _register_ said factory.

### Page URL

https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/converters-how-to?pivots=dotnet-8-0#sample-factory-pattern-converter

### Content source URL

https://github.com/dotnet/docs/blob/main/docs/standard/serialization/system-text-json/converters-how-to.md

### Document Version Independent Id

678c4f23-764a-7ccd-0d6d-159a30957477

### Article author

@gewarren

### Metadata

* ID: 5de37d79-751f-e372-e866-efde28f4908c
* Service: **dotnet-fundamentals**