docs/cookbook/redoconf-simple: a simple redoconf C++ project.

This is a little simpler than the docs/cookbook/c project, which
doesn't actually have a doc yet because there was too much to explain.
I think I might make that a follow-on cookbook chapter, for people who
have read this simple one.

I think this doc is maybe a little too long; I intended it to be
"here's what you do to get started" but it turned into "here's what you
do to get started, and why it works, in excruciating detail." Not quite
sure how to fix.

(Also updated some other parts of the docs to refer to redoconf as a
real thing now instead of a "maybe someone should write this" thing.)

docs/Roadmap.md

@@ -309,21 +309,3 @@ consider bundling redo with a particular (optional) sh implementation, and
 maybe also unix-like tools, that it will use by default.  An obvious
 candidate would be busybox, which has a win32 version called
 [busybox-w32](https://frippery.org/busybox/).
-
-
-### redoconf
-
-redo is fundamentally a low-level tool that doesn't know as much about
-compiling specific programming languages as do higher-level tools like
-[cmake](https://cmake.org/).
-
-Similarly, `make` doesn't know much about specific programming languages
-(and what it does know is hopelessly out of date, but cannot be deleted or
-updated because it would break backward compatibility with old Makefiles).
-This is why `autoconf` and `automake` were created: to automatically fill in
-the language- and platform-specific blanks, while letting `make` still
-handle executing the low level instructions.
-
-It might be useful to have a redo-native autoconf/automake-like system,
-although you can already use autoconf with redo, so this might not be
-essential.