Define part in 5b6 (#1062)

Copied directly from the style guide, probably needs
rewording/rethinking

Also perhaps `piece` needs to be defined somewhere else as well?

Co-authored-by: Lucas Garron <lgarron@worldcubeassociation.org>
Co-authored-by: ohexter <116421789+ohexter@users.noreply.github.com>
Co-authored-by: Carter Kucala <42009313+CarterKoala@users.noreply.github.com>

wca-regulations.md

@@ -199,6 +199,7 @@ Note: Because Article and Regulation numbers are not reassigned when Regulations
         - 5b5c) If more than one part with one colored face is affected, the puzzle is considered unsolved (DNF).
         - 5b5d) If one or more parts with more than one colored face are affected, the puzzle is considered unsolved (DNF).
         - 5b5e) Regulations [5b5c](regulations:regulation:5b5c) and [5b5d](regulations:regulation:5b5d) supersede [5b5a](regulations:regulation:5b5a) and [5b5b](regulations:regulation:5b5b).
+    - 5b6) Definition: A part is one or more fully connected components from a group of puzzle components that remain connected and do not move relative to each other during ordinary operation of the puzzle (e.g. a 3x3x3 center cap, a pyraminx tip, a 3x3x3 corner piece missing one face, one face of a 3x3x3 corner piece).
 
 
 ## <article-7><environment><environment> Article 7: Environment