docs: hwmon: Add an index file and rename docs to *.rst

Now that all files were converted to ReST format, rename them
and add an index.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/sht15.c

@@ -10,7 +10,7 @@
  *
  * Copyright (c) 2007 Wouter Horre
  *
- * For further information, see the Documentation/hwmon/sht15 file.
+ * For further information, see the Documentation/hwmon/sht15.rst file.
  */
 
 #include <linux/interrupt.h>