doc: UML diagrams for the nodes

Closes #2

doc_resources/nodes/class_diagram.puml

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+@startuml
+
+skinparam linestyle ortho
+left to right direction
+
+class "Supervisor" as sup{}
+class "BLE_advertiser" as ble{
+    + advertise(data)
+}
+class "Sensor" as sens{
+    + init(...)
+    + get_value()
+}
+class "Window_status" as win{}
+class "Hygrometer" as hygro{}
+class "Thermometer" as thermo{}
+class "CO2_level" as co2{}
+
+sup o-d- ble
+sup o-u- sens
+sens <|-l- win
+sens <|-u- hygro
+sens <|-u- thermo
+sens <|-r- co2
+
+@enduml