Add more wow effect to the rule example

[Omikhleia]

Just looking at the current "rule" example, I found it quite unimpressive.

• It uses the unofficial SILE logo ("à la TeX raised and lowered letters"), which I don't find that nice (and anyway this would be a demonstration of raiselower, not rules...)
• As of "rules", it just shows two very simple lines.

This PR is a proposal to have some more effects included, and even a bit of Wow! factor (as far as one can achieve something with very low-level primitives 🤣)

My assumption is that people don't really check these examples for anything else than to have a quick demonstration and showcase of SILE capabilities, so whatever goes in this example sections, it should be nice and minimally impressive.

[alerque]

Completely agree.