chore: add api docs (#1965)

docs/api/how-to.md

@@ -0,0 +1,44 @@
+# Guide to Access Memos API with OpenID
+
+Memos API supports using OpenID as the user identifier to access the API.
+
+## What is OpenID
+
+OpenID is a unique identifier assigned by Memos system to each user.
+
+When a user registers or logs in via third-party OAuth through Memos system, the OpenID will be generated automatically.
+
+## How to Get User's OpenID
+
+You can get a user's OpenID through:
+
+- User checks the personal profile page in Memos system
+- Calling Memos API to get user details
+- Retrieving from login API response after successful login
+
+Example:
+
+```
+// GET /api/v1/user/me
+
+{
+  "id": 123,
+  "username": "john",
+  "openId": "8613E04B4FA6603883F05A5E0A5E2517",
+  ...
+}
+```
+
+## How to Use OpenID to Access API
+
+You can access the API on behalf of the user by appending `?openId=xxx` parameter to the API URL.
+
+For example:
+
+```
+curl 'https://demo.usememos.com/api/v1/memo?openId=8613E04B4FA6603883F05A5E0A5E2517' -H 'Content-Type: application/json' --data-raw '{"content":"Hello world!"}'
+```
+
+The above request will create a Memo under the user with OpenID `8613E04B4FA6603883F05A5E0A5E2517`.
+
+OpenID can be used in any API that requires user identity.