Add 2.1 Abbreviations

[sakimura]

Abbreviations appear in this document without first being defined, e.g., KB-JWT.
It would make it easier for the readers if the document has an abbreviation section near the top of the document.
Proposal:
Create a new section 2.1 Abbreviation and collect all the abbreviations.

[Sakurann]

need to do after #394 is resolved to do a PR.

[bc-pi]

#394 does introduce more abbreviations where the terms are defined so might obsolete the need for a new section.

[bc-pi]

I'm gonna say #394 will sufficiently address the suggestion here.