blockout2: add page (#4945)

pages/common/blockout2.md

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+# blockout2
+
+> Tetris like game in 3D.
+> More information: <http://www.blockout.net/blockout2/>.
+
+- Start a new game:
+
+`blockout2`
+
+- Navigate the current piece on a 2D plane:
+
+`{{Up|Down|Left|Right}} arrow key`
+
+- Rotate the piece on its axis:
+
+`{{Q|W|E|A|S|D}}`
+
+- Hard drop the current piece:
+
+`Spacebar`
+
+- Pause/unpause the game:
+
+`p`