docs: w1: convert to ReST and add to the kAPI group of docs

The 1wire documentation was written with w1 developers in
mind, so, it makes sense to add it together with the driver-api
set.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/index.rst b/Documentation/index.rst
index 472b8ab..0a564f3 100644
--- a/Documentation/index.rst
+++ b/Documentation/index.rst
@@ -117,6 +117,7 @@
    target/index
    timers/index
    spi/index
+   w1/index
    watchdog/index
    virtual/index
    input/index