You are quite right, there are various things that aren't included in the scheme, that ought to be included in a complete set of documentation.
Other examples could be: release notes (though you could include them in reference), contribution guide (though it could be part of how-to) and of course, the Introduction you mention.
Mostly I think that these things probably belong outside the scheme, in the same way that say an appendix or an introduction falls outside main body of text in a book.
An introduction (in documentation) is basically marketing: a reason for someone to keep reading.
For me the scheme is not so much a complete list of every kind of thing that must be written (I think it goes without saying that you need to create useful contents tables for example) as a guide to how I should be writing and a reminder why I am writing.