​[IMP] sales: updated margins for 19.4

[meval1006]

What this PR does and why it's needed

Updates the Margins documentation for v19.4 to reflect new editing behavior on sales order Margin/Margin(%) columns and refreshes the accompanying screenshots to match the current UI.

Changes

Sales Quotations — Margins

• Documented that editing the Margin column on a sales order now automatically recalculates the Unit Price, and that both the Margin and Margin(%) columns remain editable until an invoice is created (previously documented as non-editable, automatic-calculation-only fields).
• Updated the note on the Margin(%) column to clarify it becomes read-only when the line item cost is 0, replacing the outdated statement that the columns are not editable.
• Replaced outdated screenshots throughout the page (product form, sales order margin field, settings-adjust checkboxes, and pricelist-related margin views) with updated captures reflecting the current UI.

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This saas-19.4 PR can be FWP up to master.

Task: https://www.odoo.com/odoo/my-tasks/6335005

[robodoo]