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/pidgey.asm

@@ -0,0 +1,10 @@
+; PidgeyPokedexEntry: ; 0x181cfa
+	db "TINY BIRD@" ; species name
+	dw 100, 40 ; height, weight
+
+	db   "It rapidly flaps"
+	next "its wings in the"
+	next "grass, stirring up"
+	page "a dust cloud that"
+	next "drives insect prey"
+	next "out into the open.@"