Fix documentation Makie rendering and SVG output

[ocots]

Summary

• Initialize CairoMakie without importing Makie's plotting functions into Main.
• Activate CairoMakie's SVG backend and disable PNG MIME output for Plots and Makie figures.
• Remove the page-local PNG override and fix the Markdown table formatting warning.

Closes #422.

Test plan

• julia --project=. docs/make.jl
• Makie plotting matrix: 77/77 tests
• Backend-free PlotCase tests: 28/28 tests
• Confirmed generated documentation assets are SVG with no PNG assets
• markdownlint docs/src/serialization/plotting.md

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