docs: add full documentation for io module covering Directory, File, and Stdin classes

doc/site/modules/io/stdin.markdown

@@ -1,7 +1,12 @@
 ^title Stdin Class
 
-**TODO**
+The standard input stream.
 
-## Methods
+## Static Methods
 
-**TODO**
+### **readLine**()
+
+Reads one line of input from stdin. Blocks the current fiber until a full line
+of input has been received.
+
+Returns the string of input or `null` if stdin is closed.