fix(diagram): re-enable automatic JPEG rendering through Base.show

This was previously disabled as all diagram types that were meant to
support JPEG did not actually support JPEG output. Support for some of
these diagram types has either been removed[^1] or added[^2] making the
functionality reliable again.

[^1]: https://github.com/yuzutech/kroki/releases/tag/v0.21.0
[^2]: https://github.com/yuzutech/kroki/releases/tag/v0.18.0

src/Kroki.jl

@@ -378,16 +378,6 @@ Base.showable(::MIME"image/svg+xml", ::Diagram) = true
 Base.showable(::T, diagram::Diagram) where {T <: MIME} =
   Symbol(lowercase(String(diagram.type))) ∈ get(LIMITED_DIAGRAM_SUPPORT, T(), Tuple([]))
 
-# Calling `Base.show` for JPEGs is explicitly disabled, for the time being.
-# JPEG rendering is broken for all, supposedly supported, diagram types in the
-# Kroki service. Should the support be fixed in the service, this method can be
-# easily redefined by consuming software to support JPEG in case Kroki.jl has
-# not been updated and released.
-#
-# Note that this only affects automatic rendering of `Diagram`s to JPEGs in
-# supported environments. It is still possible to use `render` to render JPEGs
-Base.showable(::MIME"image/jpeg", ::Diagram) = false
-
 """
 Defines the MIME type to be used when `show` gets called on a [`Diagram`](@ref)
 for the `text/plain` MIME type.