Old generated_*.md files aren't cleared out by make.jl

[kdayday]

docs/src/model_library/generated_*.md are written when calling docs/make.jl, but aren't deleted first. This means markdown files from old, out-of-date builds can persist, sometimes causing confusing and hard to debug errors during build if you end up with a conflict.

This update might be valuable across the Sienna package make.jl files.