Add ;:? to keybar and help

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+*View this file with results and syntax highlighting [here](https://mlochbaum.github.io/BQN/help/predicate.html).*
+
+# Question Mark (`?`)
+
+## `?`: Predicate
+[→full documentation](../doc/block.md#predicates)
+
+Follows a statement in a block, which must return 0 or 1. If it's 0, stop the current body and evaluate the next eligible one instead. Variables defined before the `?` stay if execution continues (1) but don't carry over to other bodies (0).
+
+        {0 ? 3 ; 4}
+
+        Min ← {𝕨<𝕩 ? 𝕨 ; 𝕩}
+
+        3 Min 5
+
+        4 Min 2