docs: add setup block to plotting guide to suppress image output in examples

Add @setup plot block that loads Plots and disables image/png showable
to prevent inline rendering in docs. Update render example text to note
that the ExtensionError cannot be demonstrated because Plots is loaded
by make.jl. Move the render(fig) example after the explanatory text.

Author: ocots

docs/src/guide/plotting.md

@@ -4,6 +4,11 @@
 CurrentModule = CTBase
 ```
 
+```@setup plot
+using Plots
+Base.showable(::MIME"image/png", ::Plots.Plot) = false
+```
+
 The [`CTBase.Plotting`](@ref) submodule is a generic, domain-free plotting engine.
 It manipulates a backend-agnostic **intermediate representation** (IR): a weighted
 tree of titled axes carrying series and decorations. The engine knows nothing about
@@ -178,19 +183,16 @@ a `Figure` onto an existing backend target, targeting cells by the deterministic
 leaf order (see [`leaves`](@ref)).
 
 Without a backend loaded, the fallback throws an
-[`CTBase.Exceptions.ExtensionError`](@ref):
+[`CTBase.Exceptions.ExtensionError`](@ref). This cannot be demonstrated
+in these docs because `Plots` is loaded by `make.jl` to produce the
+examples below, which causes the `CTBasePlots` extension to be active.
+
+Once `Plots` is loaded, `render(fig)` produces a Plots.jl plot:
 
 ```@example plot
-fig2 = CTBase.Plotting.Figure(CTBase.Plotting.Leaf(ax))
-try # hide
-CTBase.Plotting.render(fig2)
-catch e # hide
-showerror(IOContext(stdout, :color => false), e) # hide
-end # hide
+CTBase.Plotting.render(fig)
 ```
 
-Once `Plots` is loaded, `render(fig)` produces a Plots.jl plot.
-
 ## Leaf Traversal
 
 [`leaves`](@ref) returns the `Leaf` nodes of a layout tree in deterministic