docs: Convert the deviceio template to RST

Convert deviceiobook.tmpl to RST and incorporate it into the driver API
manual.

Like the rest of our documentation, this one could use some work.  There's
no mention of ioremap() and friends, no mention of io_read*() and friends.
But we have nice documentation for all those folks writing new drivers that
do port I/O :).

The :c:func: notation has been left off of all the read*/write* functions.
There's no kerneldoc comments for them anyway, so those links will never be
live, and writing a bunch of repetitive "read a byte from I/O memory"
comments lacks appeal.

Cc: Matthew Wilcox <willy@infradead.org>
Cc: Alan Cox <gnomes@lxorguk.ukuu.org.uk>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>