Add GlobalPool pooling docs

Author: aurorarossi

GNNLux/docs/src/api/pool.md

@@ -0,0 +1,19 @@
+```@meta
+CurrentModule = GNNLux
+CollapsedDocStrings = true
+```
+
+# Pooling Layers
+
+## Index
+
+```@index
+Order = [:type, :function]
+Pages   = ["pool.md"]
+```
+
+```@autodocs
+Modules = [GNNLux]
+Pages   = ["layers/pool.jl"]
+Private = false
+```