Docs/cookbook: build from toplevel 'test' instead of toplevel 'all'.

The cookbook examples are interesting as redo tests, but don't need to
be built in order to build redo.

Documentation/all.do

@@ -1,4 +1,4 @@
-redo-ifchange doc.list cookbook/all
+redo-ifchange doc.list
 sed 's/\.md/.1/' <doc.list |
 xargs redo-ifchange
 

Documentation/test.do

@@ -0,0 +1,4 @@
+# We don't normally build the cookbook examples, because they're
+# not really part of redo itself.  But build them when testing,
+# as a basic check that redo (and the examples) are valid.
+redo cookbook/all