split out declarative varieties into the declarative mapping section

split out declarative varieties into the declarative mapping section

specific declarative styles like those for dataclasses and attrs
should be in the more reference-oriented "declarative_mapping"
section rather than the more introduction-oriented mapping_styles.

this will also make it easier for us to add still more ways of
mapping declaratively for 2.0.

Change-Id: I2700c2c2b34db9680f9cbe6ed6197add773a6a5d