docs(API): include docstrings for Diagram methods directly

Instead of duplicating the docstrings and signatures for the
constructors of the `Diagram` type in its docstring, the constructors
should be added to the API themselves.

docs/src/api.md

@@ -9,6 +9,8 @@ CurrentModule = Kroki
 ```@docs
 Kroki
 Diagram
+Diagram(::Symbol, ::AbstractString)
+Diagram(::Symbol; ::Union{Nothing,AbstractString}, ::Union{Nothing,AbstractString})
 render
 ```
 

src/Kroki.jl

@@ -30,24 +30,6 @@ const Maybe{T} = Union{Nothing, T} where {T}
 """
 A representation of a diagram that can be rendered by a Kroki service.
 
-# Constructors
-
-```
-Diagram(type::Symbol, specification::AbstractString)
-```
-
-Constructs a `Diagram` from the `specification` for a specific `type` of
-diagram.
-
-```
-Diagram(type::Symbol; path::AbstractString, specification::AbstractString)
-```
-
-Constructs a `Diagram` from the `specification` for a specific `type` of
-diagram, or loads the `specification` from the provided `path`.
-
-Specifying both, or neither, keyword arguments is invalid.
-
 # Examples
 
 ```