Add TON Center API v3 auth details page

docs.json

@@ -73,6 +73,7 @@
                 "expanded": true,
                 "pages": [
                   "ecosystem/api/toncenter/get-api-key",
+                  "ecosystem/api/toncenter/v3-authentication",
                   {
                     "group": "API v2",
                     "openapi": {

ecosystem/api/toncenter/v3-authentication.mdx

@@ -0,0 +1,39 @@
+---
+title: "Authentication for API v3"
+---
+
+## Overview
+
+The API v3 requires an API key for all methods.
+The key can be sent either in an HTTP header or as a query parameter.
+
+To obtain an API key, see the [TON Center API key guide](/ecosystem/api/toncenter/get-api-key).
+
+| Type    | Location | Name        | Required |
+| ------- | -------- | ----------- | -------- |
+| API key | Header   | `X-API-Key` | Yes      |
+| API key | Query    | `api_key`   | Yes      |
+
+Only one of these is needed per request.
+
+## Header authentication
+
+Send the API key in the `X-API-Key` header:
+
+```bash
+curl "https://<HOST>/api/v3/masterchainInfo" \
+  -H "X-API-Key: <API_KEY>"
+```
+
+- `<HOST>` - The base URL of the TON Center API instance (`toncenter.com` for example).
+- `<API_KEY>` - The API key obtained from the [TON Center bot](https://t.me/toncenter).
+
+## Query parameter authentication
+
+Pass the key as a query parameter named `api_key`:
+
+```bash
+curl "https://<HOST>/api/v3/masterchainInfo?api_key=<API_KEY>"
+```
+
+Both forms are equivalent.