improve names for datatypes section

improve names for datatypes section

Continuing along #8362, if we look at the document here without
"this is the same text I've read for 14 years", we begin to see
that the title "Column and Data types" makes no sense at all,
is there a "column type" and a "Data type"?  I guess what I was
thinking at that time is that a type can be placed on a
Column, or it can be used whenever you have, you know, "data".
The phrase "SQL expression" wasn't discovered yet.

"SQL Datatype" is not spectacular but at least it's one term.
the new intro then is focused on the hierarchy layout so let's
name it that.

not amazing, but better than the duplicate name that made
no sense before.

Fixes: #8362
Change-Id: Iab37ef5605ec55f30284ac9a98bf7246f736675d