WW-3364 docs: describe opt-in type converters in JSON serialization

[lukaszlenart]

Fixes WW-3364

Documents the new struts.json.writer.useTypeConverters constant from apache/struts#2002 on the JSON plugin page, in a new "Using registered type converters" section after "Java records and Optional":

• enabling the constant and a before/after example with a Money converter
• which converters are used (per-type / application-wide only; superclass and interface registrations apply to subtypes; per-property mappings are not)
• natively serialized types are never converted — including BigDecimal, which is the example on the Type Conversion page
• @JSONFieldBridge precedence and the fallback when a converter fails
• a converted value is a single string, so nested includeProperties / excludeProperties patterns and member @JSON annotations no longer apply to it

Merge after apache/struts#2002; the "Since Struts 7.5.0" marker follows the ticket's fix version.

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[asf-ci]

Staged site is ready at https://struts.staged.apache.org/