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

Author: philwebb