Add dms field for sale

This module connects sales orders with the OCA Document Management System (DMS). On the standard sales order form, a new "Documents" tab appears in the same area as the other tabs (Order Lines, Other Information, etc.). Inside that tab, users see the list of document directories linked to the order and can work with the files stored there.

• The tab is only shown for orders that already exist (it is hidden while the order is still unsaved), so documents are managed on real orders.
• The sales order model is extended with the DMS field mixin, which is what provides the document directory field used in the tab.
• The module depends on sale_management for the sales order screens and on dms_field for the document management features.

Demo data also shows how the integration is meant to be set up: a DMS access group named "Everyone for Sale DMS" is created for salespeople (with create, edit and delete permissions), and a DMS field template named "Sale" is linked to the sale.order model and to the salesperson field, so each salesperson gets their own document space.

No new menus, reports, automated checks or scheduled actions are added by the code; the change is limited to the sales order form and the document access configuration.