I honestly also think it is worth mentioning introductions as an essential part. I somewhat disagree that it is outside the scope of a scheme of documentation.

An introduction to both the system goals and overall architecture provide essential context for the rest of the documentation, and can even provide the scaffold to structure it around.

It is all too common for writers of documentation (deeply embedded in the project) to forget that the person reading the documentation may have no clue what the whole system is trying to achieve (especially so in public-facing documentation... how many times do you see a link on HN to a documentation page with no explanation of what "a BUZZY GRAPHEME WRANGLER" actually is?).

Also... by your analogy, I am fairly certain that a guide to writing a technical book would dedicate a fairly considerable amount of space to the topic of a good introduction ;)