Give the Advisory Board a mission statement

Tries to tie together the various roles under
a coherent mission statement, similar to the TAG.

Based on discussions captured in
  w3c/AB-memberonly#248
  https://www.w3.org/2025/03/26-ABpurpose-minutes.html
  https://www.w3.org/2025/Talks/AC/ab-wendy-reid/slides.pdf

Author: fantasai

index.bs

@@ -872,9 +872,12 @@ Advisory Board (AB)</h4>
 <h5 id=ab-role>
 Role of the Advisory Board</h5>
 
-	The <dfn export lt="Advisory Board|AB">Advisory Board</dfn> provides ongoing guidance to the Team
+	The <dfn export lt="Advisory Board|AB">Advisory Board</dfn>
+	is elected by the Membership to advocate for the effectiveness of W3C as an organization.
+	It provides ongoing guidance to the Team
 	on issues of strategy,
 	management,
+	community engagement,
 	legal matters,
 	process,
 	and conflict resolution.