assoc: add page (#2342)

pages/windows/assoc.md

@@ -0,0 +1,15 @@
+# assoc
+
+> Display or modify file extension associations.
+
+- Display all associated filetypes:
+
+`assoc`
+
+- Display the associated filetype for a specific extension:
+
+`assoc {{.txt}}`
+
+- Modify the associated filetype for a specific extension:
+
+`assoc {{.txt}}={{txtfile}}`