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.

for the cherry-pick, also remove the misleading line about
stubs.

Fixes: #7575
Change-Id: I2700c2c2b34db9680f9cbe6ed6197add773a6a5d
(cherry picked from commit 6bfd428eea7bf712d962a0c4c6a7fb15e14955f6)