docs: add firmware protocol documentation

docs/firmware-protocol.md

@@ -0,0 +1,54 @@
+# Firmware protocol
+
+Below you should/are able to see a sequence diagram of what packets are sent when:
+
+```mermaid
+sequenceDiagram
+    loop every second
+        Tracker-->>Network: Broadcast tracker info packet
+        Network->>Server: Receive message
+
+        break when server responds
+            Server->>Tracker: Acknowledge and connect
+        end
+    end
+
+    Tracker->>Server: Tracker feature list
+    Server->>Tracker: Server feature list
+
+    loop for every sensor
+        Tracker->>+Server: Sensor info of sensor $n
+        Server->>Server: Store sensor
+        Server->>-Tracker: Acknowledge
+    end
+
+    par
+        loop every couple of seconds
+            Tracker->>Server: Battery level
+        end
+    and
+        loop every couple of seconds
+            Tracker->>Server: WiFi strength
+        end
+    and
+        loop every second
+            Server->>Tracker: Heartbeat
+            Tracker->>Server: Heartbeat
+            Server->>Server: Update last seen of tracker
+
+            break when the last time seen > 3s ago
+                Server-->Server: Remove tracker from known trackers
+            end
+        end
+    and
+        loop every couple of seconds
+            Server->>Tracker: Ping
+            Tracker->>Server: Echo
+            Server->>Server: Store latency
+        end
+    and
+        loop every tracker tick
+            Tracker->>Server: Rotation data for sensor with ID $n
+        end
+    end
+```