Resolve "Document kramdown abbreviation syntax in handbook"

Why is this change being made?

Our handbook style guide recommends that we avoid using abbreviations and acronyms. When they are used, we should make the as useful as possible. Kramdown, the Markdown extension used to write content for the handbook, supports definition of an abbreviation in the content itself. The syntax is lightweight, and automatically wraps all instances of the acronym or abbreviation in an <abbr> tag with the appropriate title property.

This change simply informs people about the syntax. We will take steps later, as documented in #4762, to automate the process and generate a global acronym definition list.

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what

Closes #6543 (closed)