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/natu.asm

@@ -0,0 +1,10 @@
+; NatuPokedexEntry: ; 0x1cd47d
+	db "LITTLE BIRD@" ; species name
+	dw 8, 40 ; height, weight
+
+	db   "It is extremely"
+	next "good at climbing"
+	next "tree trunks and"
+	page "likes to eat the"
+	next "new sprouts on"
+	next "the trees.@"