Renamed 'Buy' section to 'Companies'

Added pages for manufacturer, customer and sales orders documentation

docs/companies/supplier.md

@@ -0,0 +1,35 @@
+---
+title: Suppliers
+---
+
+## Suppliers
+
+A supplier is an external vendor of parts and raw materials.
+
+To access the supplier page, click on the "Buy" navigation tab and click on "Suppliers" option in the dropdown list.
+
+### Add Supplier
+
+Once the supplier page is loaded, click on the "New Supplier" button: the "Create new Supplier" form opens. Fill-in the supplier informations (`Company name` and `Company description` are required) and click on "Submit".
+
+!!! note "Supplier vs Manufacturer"
+	In the case the supplier is a manufacturer who sells directly to customers, you may want to enable the checkbox `is manufacturer` before submitting the form (you can also enable it later on).
+
+### Edit Supplier
+
+!!! missing "TODO"
+	This section requires further work
+
+## Supplier Parts
+
+Supplier parts are linked to a supplier and defined as purchasable items.
+
+### Add Supplier Part
+
+!!! missing "TODO"
+	This section requires further work
+
+### Edit Supplier Part
+
+!!! missing "TODO"
+	This section requires further work