Document the things

[A1Liu]

I added more endpoints, but DIDN'T DOCUMENT ANY OF THEM. Also I keep forgetting to document stuff generally tbh.

• Courses endpoint has a full query parameter to get stuff like description
• There's a "current semester" endpoint which, as one would guess, gets the current semester

Note: this is for improving the generated OpenApi documentation, NOT for documenting code. That's a separate task.

[A1Liu]

NOTE: Now that Schedge is using Javalin 5, the OpenAPI plugin requires accessor methods to work.

[A1Liu]

• Nullability using @NotNull
• Examples using OpenApiExample