Add links to Introduction.

This adds some links to key sections that introduce major
concepts that used to be in the introductory sections of the OAS.
It insures that concepts like the difference between serialized
and in-memory data, as well as the multi-document structure of OADs,
are quickly noticed in a top-to-bottom reading (rare as those may be).

Author: handrews

src/oas.md

@@ -8,9 +8,9 @@ This document is licensed under [The Apache License, Version 2.0](https://www.ap
 
 ## Introduction
 
-The OpenAPI Specification (OAS) defines a standard, language-agnostic interface to HTTP APIs which allows both humans and computers to discover and understand the capabilities of the service without access to source code, documentation, or through network traffic inspection. When properly defined, a consumer can understand and interact with the remote service with a minimal amount of implementation logic.
+The OpenAPI Specification (OAS) defines a standard, language-agnostic interface to HTTP APIs which allows both humans and computers to discover and understand the capabilities of the service without access to source code, documentation, or through network traffic inspection. When properly defined, a consumer can understand and interact with the remote service by [parsing and serializing](#parsing-and-serializing) HTTP messages to and from a [data model](#data-types) with a minimal amount of implementation logic.
 
-An OpenAPI Description can then be used by documentation generation tools to display the API, code generation tools to generate servers and clients in various programming languages, testing tools, and many other use cases.
+An [OpenAPI Description](#openapi-description-structure) (OAD) can then be used by documentation generation tools to display the API, code generation tools to generate servers and clients in various programming languages, testing tools, and many other use cases.
 
 For examples of OpenAPI usage and additional documentation, please visit [[?OpenAPI-Learn]].