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/cookbook/redoconf-simple/all.do

@@ -0,0 +1,15 @@
+# Run from the source dir.
+#
+# We'll make sure the out/ dir exists and that
+# a C++ compiler is available, then redo out/all,
+# which is implemented in the file all.od.
+#
+# Note that a "normal" project might not have an all.do
+# at all; the end user would be expected to make an
+# output dir, run ../configure, and then redo from there.
+# But we want this file to build as part of the redo
+# examples, so there needs to be a toplevel all.do in
+# each example.
+#
+. ./skip-if-no-cxx.sh
+redo-ifchange out/all