Move the section "Understanding YAML in specs" to Introduction in specs reference

Problem

The YAML specs reference is not only a reference of features. It also have key details on how to use the reference itself. The problem is that this part is one of the last parts of the reference itself and users are not reading it. The section is called "Understanding YAML in specs" and since it starts with the U character and the document is organized in alphabetical order it is the last part of it.

Proposed solution

Make the "Understanding YAML in specs" part of the introduction section of the document.