reorganize ORM Annotated Declarative into its own section

reorganize ORM Annotated Declarative into its own section

The ORM Annotated Declarative section is now very large but has been
indented under the "Declarative Table with mapped_column()" section
where it does not show up well on top level TOCs and is too deeply
nested.   Break it out into its own section following the
entire "Declarative Table" section, but also maintain a short intro
section inside of "Declarative Table" to ensure this use is still
prominent.

Change-Id: I42f4aff6ed54da249c94ddf50727f9fe3c3bd625