make geopub/leaflet a separate library

doc/dune

@@ -0,0 +1,3 @@
+(documentation
+ (package leaflet)
+ (mld_files index))

doc/index.mld

@@ -0,0 +1,17 @@
+{0 leaflet}
+
+{{:https://TODO/leaflet} leaflet} is an {{:https://ocaml.org} OCaml} library/executable to TODO.
+
+{1:api API}
+
+{!modules:
+Leaflet
+}
+
+{1:private_api Private API}
+
+You shouldn't have to use any of these modules, they're used internally only.
+
+{!modules:
+TODO
+}