Add example of line item options using donation purchasable

[lukeholder]

Description

Maybe add an example of options usage with a donation purchasable here: https://craftcms.com/docs/commerce/3.x/orders-carts.html#line-item-options-and-notes

Or add an option to the donation example: https://craftcms.com/docs/commerce/3.x/donations.html#adding-the-donation-to-the-cart

Context: craftcms/commerce#2115