sphinx: rename top level document in each manual

It is more common to call the top level document index.rst. This is
what this patch is doing, along with all required references fixup.

(From yocto-docs rev: 2cea7fbba9210479fc0387d7e1b80da9885558f0)

Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/index.rst

@@ -0,0 +1,31 @@
+.. SPDX-License-Identifier: CC-BY-SA-2.0-UK
+
+==============================
+Yocto Project Reference Manual
+==============================
+
+|
+
+.. toctree::
+   :caption: Table of Contents
+   :numbered:
+
+   ref-system-requirements
+   ref-terms
+   ref-release-process
+   migration
+   ref-structure
+   ref-classes
+   ref-tasks
+   ref-devtool-reference
+   ref-kickstart
+   ref-qa-checks
+   ref-images
+   ref-features
+   ref-variables
+   ref-varlocality
+   faq
+   resources
+   history
+
+.. include:: /boilerplate.rst