finished #stdio and #redirect sections, added xrefs in appendix B, added new example file

examples/stdout.py

@@ -0,0 +1,17 @@
+import sys
+
+class RedirectStdoutTo:
+    def __init__(self, out_new):
+        self.out_new = out_new
+
+    def __enter__(self):
+        self.out_old = sys.stdout
+        sys.stdout = self.out_new
+
+    def __exit__(self, *args):
+        sys.stdout = self.out_old
+
+print('A')
+with open('out.log', mode='w', encoding='utf-8') as a_file, RedirectStdoutTo(a_file):
+    print('B')
+print('C')