feat(checkout): CHECKOUT-7756 Update docs around item-level discount

[valentindellangela]

[DEVDOCS-]

{Ticket number or summary of work}

What changed?

Provide a bulleted list in the present tense

• V3/Discounts API can now be used to add item-level discount in addition to cart level discount
• You cannot apply a manual discount against a specific line item if a coupon/automatic promotion is already applied to this item

Anything else?

Add related PRs, salient notes, ticket numbers, etc.

• We can't publish this PR until the project has been completed as it involves multiple teams working together - That's why I created a Draft PR

Testing?

ping @bigcommerce/team-checkout @bigcommerce/dev-docs-collaborators

[valentindellangela]

I updated this PR to include this new limitation

You cannot apply a manual discount against a specific line item if a coupon/automatic promotion is already applied to this item

[bc-traciporter]

Made some editorial suggestions

[slsriehl]

@valentindellangela Please reopen when ready using successor branch https://github.com/bigcommerce/docs/tree/CHECKOUT-7756.