Document @Bean 'lite' mode vs @Configuration

Rework the reference documentation to better distinguish the differences
between @Bean methods used in @Comonent vs @Configuration classes. The
'Using the @Bean annotation' section now only covers concepts applicable
when using @Bean methods in @Configuration _or_ @Component classes.
Information only applicable to @Configuration classes has been moved to
a new  'Using the @Configuration annotation' section.

An additional sidebar section attempts to explain the differences
between the two approaches.

Issue: SPR-9425