Add ability to generate internal API documentation using Doxygen.

lib/read.h

@@ -1,15 +1,17 @@
 // SPDX-License-Identifier: GPL-2.0
 #ifndef READ_H
 #define READ_H
-/*
+/**
+ * \file
+ *
  * Provides a function to read a numeric value.
  *
- * Copyright (C) 2020 Martin Whitaker.
+ * Copyright (C) 2020-2022 Martin Whitaker.
  */
 
 #include <stdint.h>
 
-/*
+/**
  * Returns an unsigned numeric value entered on the keyboard. Echoes the
  * input to the display field located at (row,col), limiting it to field_width
  * characters. If the entered value is prefixed by "0x", assumes base 16,