Alright, here is a first draft!

It's currently in Org, I'll export it to Markdown later.

It's much longer than I expected.  About 5000 words!  I guess some parts could
go, but I'm not sure which.  Feedback would be very much appreciated here.

Let me know if it's approachable enough and if you find any missing/tangled
parts in the progression.

I would also love feedback about the section titles.  Some could be made more
explicit I suppose.

I'm not satisfied with the advanced example: I took ~mg~ because it has a lot of
typical customization (snippets, inputs, arguments) but it does not have
- in-input sources
- %build-inputs
- propagated-inputs

I've made up a "my-mg" package but it relies on made-up inputs, so it won't
build.
Better suggestions anyone?