Improve HtmlFlow-Datastar integration documentation

[PauloCarvalho13]

Improve the HtmlFlow-Datastar integration documentation to clarify how nested signals and non-primitive values work.
HtmlFlow-Datastar examples illustrating the expected Datastar output:

data class Person(
    val name: String,
    val age: Int,
)

div {
    val (person) = dataSignals(
        "person" to Person("John Doe", 18),
    )

    input {
        dataBind(person.on(Person::name))
    }

    p {
        dataText { +person.on(Person::age) }
    }
}

This should initialize person as an object, equivalent to:

<div data-signals="{person: {name: 'John Doe', age: 18}}">
  <input data-bind="person.name">
  <p data-text="$person.age"></p>
</div>

A string that looks like an object must remain a string:

div {
    dataSignals(
        "person" to "{name: 'John Doe', age: 18}",
    )
}

This is equivalent to a string-valued signal, not a nested object:

<div data-signals="{person: '{name: \'John Doe\', age: 18}'}"></div>

It should therefore not be used with nested bindings such as dataBind("person.name").
Object assignments should follow the same value semantics as initialization:

data class Person(
    val name: String,
    val age: Int,
)

div {
    val (person, otherPerson) = dataSignals(
        "person" to Person("John Doe", 18),
        "otherPerson" to Person("Jane Doe", 21),
    )

    button {
        dataOn(Click) {
            person.setValue(Person("Jane Doe", 21))
        }
        text("Replace with a value")
    }

    button {
        dataOn(Click) {
            person.setValue(otherPerson)
        }
        text("Copy from another signal")
    }
}

The first assignment represents a literal object value:

$person = {name: 'Jane Doe', age: 21}

The second represents an expression evaluated by Datastar:

$person = $otherPerson

The documentation should explicitly cover this distinction, including nested objects, arrays, null, and correctly escaped strings.
Related integration issues:

• Enforce typed structured signal values HtmlFlow-Datastar#16
• Serialize signal values consistently HtmlFlow-Datastar#17,