Rename all files in data/pokedex/entries/

Some people suggested this would be a good idea, and make management of
the files easier. I'm inclined to agree.

data/pokedex/entries/articuno.asm

@@ -0,0 +1,10 @@
+; ArticunoPokedexEntry: ; 0x1cc632
+	db "FREEZE@" ; species name
+	dw 507, 1220 ; height, weight
+
+	db   "Legendary bird"
+	next "#MON. As it"
+	next "flies through the"
+	page "sky, it cools the"
+	next "air, causing snow"
+	next "to fall.@"