docs: add swagger ui

Akzeptanzkriterien

• Hinzugefügt in pom.xml
• Im Browser aufrufbar
• Korrekte Generierung (so weit wie möglich..)

added Backend label

assigned to @lb210

mentioned in commit c5a74cf4

mentioned in commit 2dfa6f79

mentioned in commit e8b2db2c

mentioned in commit 8766fb59

mentioned in merge request !24 (merged)

mentioned in commit cc7c4379

mentioned in commit e33fdd21

closed