WCAG 3.0 -> WCAG 3

[shawna-slh]

To help with documentation in the future, please use "WCAG 3" (instead of "WCAG 3.0") in most places.

Use "WCAG 3" in most cases; e.g., WCAG 3 Introduction.
Can use WCAG 3.0 for references to the TR doc.
https://www.w3.org/WAI/EO/wiki/Style#Referring_to_xxAG_.28WCAG_2.3B_singular.29

Let me know if questions or suggestions for amending that. Thanks!