Build examples in site CI

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Fixes #16 Fixes #17

This completely overhauls the site builder again. It still does our own thing by running Jekyll our own way, but I move our way to Nix. Now we have access to not just Jekyll but anything else we want in the same environment.

• [x] Copy example sources from SILE repo into site repo.
• [x] Use SILE (what else) to render the examples to PDF.
• [x] Use ImageMagick to covert the first page of each PDF to a PNG.
• [x] Install all fonts used in examples into CI environment.
• [x] Fix broken examples.

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Extra thanks to @ilkecan for the help getting Jekyll setup equivalent to GitHub Pages running in Nix and @doronbehar for the reminders that I might like Nix if I actually tried it. I do.

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Not all the examples are working, but I think I fixed most of the ones where the trouble was in the example. A few are still showcases of really poor handling, but they work at least as well or better than when the examples were in the SILE repository.