[docs] made structure for user docs combining manual and existing help

docs/docs.user.buying.md

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+---
+{
+	"_label": "Buying"
+}
+---
+If your business involves physical goods, buying is on of your core business activities. Your suppliers are as important as your customers and they must be provided with as much accurate information as possible.
+
+Buying in right amounts at right quantities can affect your cash flow and profitability.
+
+ERPNext contains a set of transactions that will make your buying as efficient and seamless as possible.