Move URI/URL resolution under OAD Structure

Almost all of our guidance on parsing and resolving OADs is under
the section "OpenAPI Description Structure", _except_ for the
parts on resolving relative OAD URI and API URL references.
Those two sections are further down, after a lengthy discussion
of data types.

This moves (without any changes except heading levels) those
URI/URL resolution sections up with all of the other parsing
guidance.  I have placed them before the "Implicit Connections"
section because those connections are "Implicit" in contrast
to URI references, which are explicit.

This puts all of the parsing guidance in one place, and properly
contextualizes "Implicit Connections" instead of introducing
them before the far-more-common URI connections.