「🎉」 init: started writing doc for auth api

doc/auth/login.md

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+# Login
+
+Abailable endpoints:
+- `/login`
+- `/login/google`
+- `/login/google/callback`

doc/auth/register.md

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+# Register
+
+Available endpoints:
+- `/register`
+- `/register/google`
+- `/register/google/callback`
+
+Common return:
+- 500 with response 
+```json
+{
+    "error": "Internal server error"
+}
+```
+
+## `/register`
+
+Input needed :
+```json
+{
+    "user": "<string>",
+    "password": "<string>"
+}
+```
+
+Can return: 
+- 200 with response and cookie in header
+```json
+{
+    "msg": "Register successfully"
+}
+```
+- 400 with response
+```json
+{
+    "error": "<corresponding error>"
+}
+```
+
+## `/register/google`
+
+Does not take input
+
+Always return:
+- redirect to the google auth url
+
+## `/register/google/callback`
+
+inputs are filled by google
+
+Can return:
+- 400 with response