Add dcoumentation for API (#18)

apps/docs/api-reference/introduction.mdx

@@ -0,0 +1,24 @@
+---
+title: Introduction
+description: "Fundamental concepts of Usend's API."
+---
+
+## Base URL
+
+Unsend's API is built on REST principles and is served over HTTPS. To ensure data privacy, unencrypted HTTP is not supported.
+
+The Base URL for all API endpoints is:
+
+```sh Terminal
+https://app.unsend.dev/api/
+```
+
+## Authentication
+
+Authentication to Usend's API is performed via the Authorization header with a Bearer token. To authenticate, you need to include the Authorization header with the word Bearer followed by your token in your API requests like so:
+
+```sh Terminal
+Authorization: Bearer us_12345
+```
+
+You can create a new token/API key under your Unsend [Developer Settings](https://app.unsend.dev/api-keys).