docs: EXPOSED-1080 Add docs for non-null transform

[Vik Nikolova (vnikolova)]

Description

Add a new section in Column transformation for the .nonNullTransform() function.

Additionally:

• convert .topic file to MD.
• split up content into more sections (Basic usage, Reuse a transformation).
• align content with the KED style guide.
• add explanations (bullet points) to code block examples.

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Type of Change

• Bug fix
• New feature
• Documentation update

Related Issues

EXPOSED-1080 Documentation for Nullable column transform should be able to produce non-null type

[Oleg Babichev (obabichevjb)]

Or even like that:

fun Column<LocalTime>.mealTransform(): Column<Meal> =
    transform(MealTimeTransformer())

object Meals : Table() {
    val mealTime: Column<Meal> = time("meal_time").mealTransform()
}