Added Use Discounts in Checkout documentation

Editor assessment

A straightforward docs addition. The subject identifies the new page — using discounts in checkout — with enough specificity to be useful in release notes, though 'added' should be imperative 'add', and the mid-sentence capitalization 'Use Discounts in Checkout' mirrors the page title at the cost of sentence-case consistency; quoting it as a title or normalizing case would read cleaner. No scope, matching repo convention; no body, acceptable for a self-contained guide, though naming the covered scenarios (discount codes, automatic discounts, gift cards?) would aid discovery. No breaking markers, correctly, and zero release impact since docs commits should not trigger versioning. Structurally valid and easily parsed. Competent and clear, held back only by the habitual past-tense verb and slightly awkward title casing.