I think we may need one more section between these two that covers scriptlets as an optional step:
“Extending the build”
Alternatively, we could home it under “Additional features” in the TOC and link off to it inside “Further customisations” in each Creating a snap guide. This is how the existing language guides work.
I’m not a big fan of putting things in front of all developers that will only apply to a fraction. It’s unnecessary cognitive load. But I can’t think of a better place for it that would still be discoverable by those who need it.