[IMP] Inventory: units of measure - group products in POS

[erjer-odoo]

What this PR does and why it's needed

In response to feedback, this PR documents the Group Products in POS feature for units of measure, which lets products in a UoM category be grouped together at the POS register and on the customer display instead of being listed individually. It also refreshes the surrounding UoM documentation and screenshots for accuracy.

Changes

Units of measure

• Added a new example and screenshots showing a sales order and delivery in a different unit of measure.
• Updated screenshots to reflect the current UI.

Group units of measure in the POS app

• Added a new "Group units of measure in the POS app" section explaining that products can be grouped by unit of measure at the POS register and on the customer display to reduce clutter when a customer orders multiple units of the same product.
• Documented the steps to enable this.

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This 17.0 PR can be FWP up to 18.0 — there are changes on later branches that will warrant their own separate PRs.

[robodoo]

[erjer-odoo]

@robodoo fw=no

[robodoo]

Disabled forward-porting.

[erjer-odoo]

@robodoo up to 18.0

[robodoo]

Forward-porting to '18.0'. Re-enabled forward-porting (you should use fw=default to re-enable forward porting after disabling).