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/kernel-dev/kernel-dev-advanced.rst

@@ -474,7 +474,7 @@ supported kernel type.
 This section overviews the BSP description structure, the aggregation
 concepts, and presents a detailed example using a BSP supported by the
 Yocto Project (i.e. BeagleBone Board). For complete information on BSP
-layer file hierarchy, see the :doc:`../bsp-guide/bsp-guide`.
+layer file hierarchy, see the :doc:`../bsp-guide/index`.
 
 Description Overview
 ~~~~~~~~~~~~~~~~~~~~

documentation/kernel-dev/kernel-dev-common.rst

@@ -502,7 +502,7 @@ your layer in the following area:
 .. note::
 
    If you are working on a new machine Board Support Package (BSP), be
-   sure to refer to the :doc:`../bsp-guide/bsp-guide`.
+   sure to refer to the :doc:`../bsp-guide/index`.
 
 As an example, consider the following append file used by the BSPs in
 ``meta-yocto-bsp``:

documentation/kernel-dev/kernel-dev-intro.rst

@@ -79,9 +79,9 @@ facilitate the process of working with the kernel recipes. If you find
 you need some additional background, please be sure to review and
 understand the following documentation:
 
--  :doc:`../brief-yoctoprojectqs/brief-yoctoprojectqs` document.
+-  :doc:`../brief-yoctoprojectqs/index` document.
 
--  :doc:`../overview-manual/overview-manual`.
+-  :doc:`../overview-manual/index`.
 
 -  :ref:`devtool
    workflow <sdk-manual/sdk-extensible:using \`\`devtool\`\` in your sdk workflow>`