They're either embedded in the text or as an appendix. Its not really a hard fast 6 pages exactly thing, more of a guideline. You want it to have depth but not be too long. Thats pretty much it. For more details: I'd say 1 pager is an idea. 2 pager is idea with the why and some details fleshed out. 6 pager is idea, why/how/people, design docs, stats/analysis and maybe even some tech choices.

I thought it was a good model look into and plan projects. IMO, programming should be / is only 20-30% of a softdev's job, the rest should be planning (and probably fixing bugs afterwards haha).

Source: Former Amazon dev.

I don't - there are plenty of ways to get visual ideas/concepts into the document. You can refer them to the appendix or embed it in. Appendixes are often quite large on complex topics.

The big benefit is that meetings start with people reading the documents and thinking about them - you'll see people making notes in the margins, etc. Only after everyone has had a chance to read the doc do you start discussing it, so you save a lot of time answering questions that are answered in the doc.