Fix broken Kotlin examples in reference documentation

[dmitrysulman]

There are multiple issues related to the Kotlin examples included in the Antora docs of the spring-boot-docs module.

1. All Kotlin examples on the Development-time Services page are located in the wrong package org.springframework.boot.docs.features.testcontainers.atdevelopmenttime.devtools instead of org.springframework.boot.docs.features.devservices.testcontainers.atdevelopmenttime.devtools. As a result, none of the Kotlin examples from this page are picked up by Antora:

2. The MyContainers.kt example file is empty, unlike its Java counterpart:
   

   spring-boot/spring-boot-project/spring-boot-docs/src/main/kotlin/org/springframework/boot/docs/features/testcontainers/atdevelopmenttime/importingcontainerdeclarations/MyContainers.kt

   Lines 17 to 20 in af926e2

   	package org.springframework.boot.docs.features.testcontainers.atdevelopmenttime.importingcontainerdeclarations
   	class MyContainers {
   	}

3. The Advanced Native Images Topics page contains an include-code::Nested[] block:

spring-boot/spring-boot-project/spring-boot-docs/src/docs/antora/modules/reference/pages/packaging/native-image/advanced-topics.adoc

Lines 9 to 17 in 4aff06a

	Reflection hints are automatically created for configuration properties by the Spring ahead-of-time engine.
	Nested configuration properties which are not inner classes, however, *must* be annotated with javadoc:org.springframework.boot.context.properties.NestedConfigurationProperty[format=annotation], otherwise they won't be detected and will not be bindable.
	include-code::MyProperties[]
	where `Nested` is:
	include-code::Nested[]

The Java version of the Nested class is used directly as an example. However, the Kotlin version of the same class was added in the Kotlin 2.1.0 migration PR only to make the code compile, as it is required by MyPropertiesKotlin.kt.

Since both classes share the same fully qualified name, Antora also picks up the Kotlin version and displays it on the page:

This is what it looks like when the Kotlin version is shown:

This PR fixes these issues by:

• Moving all Kotlin examples from org.springframework.boot.docs.features.testcontainers.atdevelopmenttime.devtools to org.springframework.boot.docs.features.devservices.testcontainers.atdevelopmenttime.devtools.
• Moving the Nested.kt file to a nested subpackage to prevent it from being picked up by Antora.
• Adding a proper Kotlin Testcontainers configuration example to the MyContainers.kt file.
• Removing the SomeController.kt file, since it's unused (another SomeController.kt exists in the parent package and is the one actually referenced).