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

docs/docs.user.selling.md

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+---
+{
+	"_label": "Selling"
+}
+---
+Selling is that the communication that happens with the customer the prior to and during the sale. You might be managing all the communication yourself or you may have a small team of sales people to handle this. ERPNext helps you track the communication leading up to the sale, by keeping all your documents in an organized and searchable manner.
+
+ERPNext helps you track business **Opportunities** from **Leads** and **Customers**, send them **Quotations** and make confirmed **Sales Orders**.