*: Set PlantUML background color to white (#419)

Night owls had a hard time reading the diagrams included in these specs.
A white background ensures that all text is readable in dark mode as well.

connections/conn-upgrade.plantuml

@@ -1,5 +1,7 @@
 @startuml
 
+skinparam backgroundColor white
+
 entity Initiator
 entity Responder
 

connections/multistream.plantuml

@@ -1,5 +1,7 @@
 @startuml
 
+skinparam backgroundColor white
+
 entity Initiator
 entity Responder
 Initiator -> Responder: Open connection

relay/circuit-v2.plantuml

@@ -0,0 +1,30 @@
+@startuml
+participant A
+participant R
+participant B
+
+skinparam backgroundColor white
+skinparam sequenceMessageAlign center
+
+== Reservation ==
+
+A -> R: [hop] RESERVE
+R -> A: [hop] STATUS:OK
+
+hnote over A: Reservation timeout approaching.
+hnote over A: Refresh.
+
+A -> R: [hop] RESERVE
+R -> A: [hop] STATUS:OK
+
+hnote over A: ...
+
+== Circuit Establishment ==
+
+B -> R: [hop] CONNECT to A
+R -> A: [stop] CONNECT from B
+A -> R: [stop] STATUS:OK
+R -> B: [hop] STATUS:OK
+
+B <-> A: Connection
+@enduml