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/psyduck.asm

@@ -0,0 +1,10 @@
+; PsyduckPokedexEntry: ; 0x182cee
+	db "DUCK@" ; species name
+	dw 207, 430 ; height, weight
+
+	db   "The only time it"
+	next "can use its psy-"
+	next "chic power is when"
+	page "its sleeping brain"
+	next "cells happen to"
+	next "wake.@"