feat: make public API available without Kroki prefix

Apart from the string literals, the `Diagram` type and `render` function
make up the main public API of the package. The string literals are
already `export`ed, but this type and function were not fully exposed
publicly. To make it easier to support scenarios that require the use of
these explicitly, they are now `export`ed as well.

src/Kroki.jl

@@ -21,6 +21,8 @@ using .Documentation
 include("./kroki/service.jl")
 using .Service: ENDPOINT
 
+export Diagram, render
+
 """
 A representation of a diagram that can be rendered by a Kroki service.