Add use case diagram and documentation

Added a PlantUML use case diagram and a markdown file describing the main use cases for the cinema system, including seat reservation, booking, cancellation, conversion, ticket status check, and statistics display for different user roles.

plantuml/usecase-diagramm.puml

@@ -0,0 +1,36 @@
+@startuml
+left to right direction
+
+:Inhaber: as owner #ffaaaa;line:red
+:Mitarbeiter: as employee #aaffaa;line:green
+:Kunde: as customer #aaaaff;line:blue
+
+(Programmübersicht anschauen) as overview
+(Sitzplatz reservieren) as reserve
+(Sitzplatz buchen) as book
+(Sitzplatzreservierung stornieren) as cancel
+(Sitzplatzreservierung\nin Buchung umwandeln) as convert
+(Kinostatistik anzeigen) as stats_cinema #ffaaaa
+'(Filmstatistik anzeigen) as stats_movie #ffaaaa
+
+(Ticketstatus prüfen) as check #aaffaa;line.dotted
+
+customer --> overview
+customer --> reserve
+customer --> book
+customer --> cancel
+customer --> convert
+
+'employee --> overview
+'employee --> reserve
+'employee --> book
+'employee --> cancel
+'employee --> convert
+employee --|> customer
+
+employee --> check
+
+owner --> stats_cinema
+'owner --> stats_movie
+
+@enduml