docs: virtual: add it to the documentation body

As files are getting converted to ReST, add them to the
documentation body.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
diff --git a/Documentation/index.rst b/Documentation/index.rst
index 68ae2a4..2df5a3d 100644
--- a/Documentation/index.rst
+++ b/Documentation/index.rst
@@ -115,6 +115,7 @@
    target/index
    timers/index
    watchdog/index
+   virtual/index
    input/index
    hwmon/index
    gpu/index