「📝」 doc: added doc to this

doc/user/ping.md

@@ -0,0 +1,41 @@
+# ping
+
+Available endpoints:
+- POST `/ping`
+- GET `/ping/:userId`
+
+Common return:
+- 500 with response 
+```json
+{
+    "error": "Internal server error"
+}
+```
+
+## POST `/ping`
+
+Used to send a ping and update the lastSeenTime (can be used for activity time)
+
+Input needed : just need a valid token
+
+Can return:
+- 200
+```json
+{
+    "msg": "last seen time updated successfully"
+}
+```
+
+## GET `/ping/:userId`
+
+Used to retrive the lastSeenTime of a user
+
+Input needed : just need a valid token
+
+Can return:
+- 200
+```json
+{
+    "lastSeenTime": "<last seen time>"
+}
+```