feat(docs): Create taxes guide

docs/content/docs/developer-guide/taxes.md

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+---
+title: "Taxes"
+showtoc: true
+---
+# Taxes
+
+Most e-commerce applications need to correctly handle taxes such as sales tax or value added tax (VAT). In Vendure, tax handling consists of:
+
+* **Tax categories** Each ProductVariant is assigned to a specific TaxCategory. In some tax systems, the tax rate differs depending on the type of good. For example, VAT in the UK has 3 rates, "standard" (most goods), "reduced" (e.g. child car seats) and "zero" (e.g. books).
+* **Tax rates** This is the tax rate applied to a specific tax category for a specific [Zone]({{< relref "zone" >}}). E.g., the tax rate for "standard" goods in the UK Zone is 20%.
+* **Channel tax settings** Each Channel can specify whether the prices of produce variants are inclusive of tax or not, and also specify the default Zone to use for tax calculations.
+* **TaxZoneStrategy** Determines the active tax Zone used when calculating what TaxRate to apply. By default, it uses the default tax Zone from the Channel settings.
+* **TaxLineCalculationStrategy** This determines the taxes applied when adding an item to an Order. If you want to integrate a 3rd-party tax API or other async lookup, this is where it would be done.
+
+## API conventions
+
+In the GraphQL API, any type which has a taxable price will split that price into two fields: `price` and `priceWithTax`. This pattern also holds for other price fields, e.g.
+
+```graphql
+query {
+  activeOrder {
+    ...on Order {
+      lines {
+        linePrice
+        linePriceWithTax
+      }
+      subTotal
+      subTotalWithTax
+      shipping
+      shippingWithTax
+      total
+      totalWithTax
+    }
+  }
+}
+```
+
+In your storefront you can therefore choose whether to display the prices with or without tax, according to the laws and conventions of the area in which your business operates.
+
+## Calculating taxes on OrderItems
+
+When a customer adds an item to the Order, the following logic takes place:
+
+1. The price of the OrderItem, and whether or not that price is inclusive of tax, is determined according to the configured [OrderItemPriceCalculationStrategy]({{< relref "order-item-price-calculation-strategy" >}}).
+2. The active tax Zone is determined based on the configured [TaxZoneStrategy]({{< relref "tax-zone-strategy" >}}).
+3. The applicable TaxRate is fetched based on the ProductVariant's TaxCategory and the active tax Zone determined in step 1.
+4. The `TaxLineCalculationStrategy.calculate()` of the configured [TaxLineCalculationStrategy]({{< relref "tax-line-calculation-strategy" >}}) is called, which will return one or more [TaxLines]({{< relref "/docs/graphql-api/shop/object-types" >}}#taxline).
+5. The final `priceWithTax` of the OrderItem is calculated based on all the above.
+
+## Calculating taxes on shipping
+
+The taxes on shipping is calculated by the [ShippingCalculator]({{< relref "shipping-calculator" >}}) of the Order's selected [ShippingMethod]({{< relref "shipping-method" >}}).
+
+## Configuration
+
+This example shows the configuration properties related to taxes:
+
+```TypeScript
+export const config: VendureConfig = {
+  taxOptions: {
+    taxZoneStrategy: new DefaultTaxZoneStrategy(),
+    taxLineCalculationStrategy: new DefaultTaxLineCalculationStrategy(),
+  },
+  orderOptions: {
+    orderItemPriceCalculationStrategy: new DefaultOrderItemPriceCalculationStrategy()
+  }
+}
+
+```