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).

apps/docs/api-reference/openapi.json

@@ -106,6 +106,18 @@
     },
     "/v1/emails/{emailId}": {
       "get": {
+        "parameters": [
+          {
+            "schema": {
+              "type": "string",
+              "minLength": 3,
+              "example": "1212121"
+            },
+            "required": true,
+            "name": "emailId",
+            "in": "path"
+          }
+        ],
         "responses": {
           "200": {
             "description": "Retrieve the user",