Add documentation for the API of the SLO Server

Maybe this can be done via FastAPI or simple in the corresponding README.

added To Do label

added Master Project 20/21 Theodolite Kotlin modul/docs modul/theodolite labels

changed the description

mentioned in issue #206 (closed)

changed milestone to %0.5.0

mentioned in merge request !123 (merged)

added In Progress label and removed To Do label

closed

removed In Progress label