docs: adds overview and main README

Author: iOvergaard

16/umbraco-cms/SUMMARY.md

@@ -183,6 +183,9 @@
   * [Extension Conditions](customizing/extending-overview/extension-conditions.md)
   * [Custom Extension types](customizing/extending-overview/custom-extension-type.md)
 * [Foundation](customizing/foundation/README.md)
+  * [Fetching Data](customizing/foundation/fetching-data/README.md)
+    * [Fetch API](customizing/foundation/fetching-data/fetch-api.md)
+    * [HTTP Client](customizing/foundation/fetching-data/http-client.md)
   * [Working with Data](customizing/foundation/working-with-data/README.md)
     * [Repositories](customizing/foundation/working-with-data/repositories.md)
     * [Context API](customizing/foundation/working-with-data/context-api.md)

16/umbraco-cms/customizing/foundation/fetching-data/README.md

@@ -0,0 +1,15 @@
+# Fetching Data
+
+Learn how to request data when extending the Backoffice.
+
+## [Fetch API](fetch-api.md)
+
+The Fetch API is a modern way to make network requests in JavaScript. It provides a more powerful and flexible feature set than the older XMLHttpRequest.
+
+## [HTTP Client](http-client.md)
+
+The HTTP Client is a wrapper around the Fetch API that provides a more convenient way to make network requests. It handles things like request and response parsing, error handling, and retries.
+
+## [Working with Data](../working-with-data/README.md)
+
+Once you have the data using one of the methods above, you can read more about how to work with it here.