Add redirect for wg-async adoption by lang

If WG-async is to be adopted by lang, let's add a redirect from where
its page previously appeared on our website to where it will now.

src/redirect.rs

@@ -38,6 +38,7 @@ static PAGE_REDIRECTS: &[(&str, &str)] = &[
     ),
     ("governance/teams/language-and-compiler", "governance#teams"),
     ("governance/teams/operations", "governance#teams"),
+    ("governance/wgs/wg-async", "governance/teams/lang#team-wg-async"),
     // miscellaneous
     ("governance/teams", "governance#teams"),
     ("governance/wgs", "governance#working-groups"),