docs: rearrange user guide (#2127)

* added lists guide

* restructure existing guides

* added new pages after restructuring

* added blank pages + rearranged sidebar

docs/content/user-guide/gift-cards/index.md

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+---
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+---
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+# Gift Cards Overview
+
+In this document, you’ll learn about how Gift Cards in Medusa work.
+
+## Overview
+
+In Medusa, there’s one main gift card. A gift card can have multiple denominations. For example, a Gift Card can have the amounts of $20 and $50, and customers can buy one of these denominations.
+
+You can also create custom gift cards in Medusa. These cards can be created and sent directly to a customer’s email.
+
+## Learn More About Gift Cards
+
+- [Manage Gift Cards](./manage.mdx)
+- [Manage Custom Gift Cards](./custom.md)