mdBook doesn't understand extended Markdown abbreviation syntax

[Dennis Marttinen]

RFC-0002 uses some abbreviations, which aren't supported by mdBook's CommonMark specification. In HackMD, this syntax generates tooltips with the abbreviation expanded, but both on GitHub and in the mdBook the end result looks like this:

So we might want to either look into implementing that support (unlikely to get support, since CommonMark aims to be highly compatible) or remove the use of the abbreviation syntax.