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/tentacruel.asm

@@ -0,0 +1,10 @@
+; TentacruelPokedexEntry: ; 0x1b8337
+	db "JELLYFISH@" ; species name
+	dw 503, 1210 ; height, weight
+
+	db   "When its 80 feel-"
+	next "ers absorb water,"
+	next "it stretches to"
+	page "become like a net"
+	next "to entangle its"
+	next "prey.@"