Document `@SpringExtensionConfig` in the reference manual

[sbrannen]

Overview

In 7.0 we introduced @SpringExtensionConfig which is documented in Javadoc, but we still need to document it in the following sections in the reference manual.

• SpringExtension for JUnit Jupiter
• Spring JUnit Jupiter Testing Annotations

Related Issues

• Use test-method scoped ExtensionContext in the SpringExtension #35697