Fix documentation Makie backend loading and SVG figure generation

[ocots]

Problem

The documentation build fails in the Makie plotting examples because CTBase.Plotting.MakieBackend is not available when Makie.plot(sol) is evaluated. The VitePress build also prefers PNG over SVG for plot objects.

Expected outcome

• The documentation build executes all Makie examples successfully.
• CairoMakie is activated with the SVG backend.
• PNG output is disabled for Plots and Makie figures so generated documentation assets are SVG.
• The existing plotting and PlotCase tests remain passing.

Verification

julia --project=. docs/make.jl