More clearly document normative vs non-normative content

I think it would be good to make it clear somehow what is normative vs non-normative. For example, notes are non-normative, but what else? How about the appendices? See C and C++ call these out in some places.

Are intro sections normative? I feel like we've been fairly inconsistent here.

I think it would be good if the introduction chapter called this out clearly, and then figure out sections that need to be called out one way or the other. The authoring guide should also provide some guidance on how to do all this.

Start by reading the introduction chapter and the authoring guide, then review how notes, appendices, and intro sections are currently treated. Identify the sections needing explicit normative or non-normative guidance and update the introduction and authoring guidance so the distinction and authoring rules are clear.