Date time input - guidance

[mi6-577]

This ticket is purely to create guidance and imagery for the datetime input component. Component has been designed and developed under #1636

What component is this related to?

Datetime input

Why do we need it?

Datetime input has been developed, but doesn't appear on the guidance site because there's the guidance hasn't been added (the guidance for this has been written but the images might need updating to have light/dark mode).

This will need a Component Gallery image too.

🚨 Urgency (low, medium or high)

High - users won't know how to use the component without guidance.

[mi6-577]

Once #1614 has been done, this will be a near copy but for datetime input