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/index.rst

@@ -14,7 +14,7 @@ Welcome to the Yocto Project Documentation
    :maxdepth: 1
    :caption: Introduction and Overview
 
-   Quick Build <brief-yoctoprojectqs/brief-yoctoprojectqs>
+   Quick Build <brief-yoctoprojectqs/index>
    what-i-wish-id-known
    transitioning-to-a-custom-environment
    Yocto Project Software Overview <https://www.yoctoproject.org/software-overview/>
@@ -25,15 +25,15 @@ Welcome to the Yocto Project Documentation
    :maxdepth: 1
    :caption: Manuals
 
-   Overview and Concepts Manual <overview-manual/overview-manual>
-   Reference Manual <ref-manual/ref-manual>
-   Board Support Package (BSP) Developer's guide <bsp-guide/bsp-guide>
-   Development Tasks Manual <dev-manual/dev-manual>
-   Linux Kernel Development Manual <kernel-dev/kernel-dev>
-   Profile and Tracing Manual <profile-manual/profile-manual>
-   Application Development and the Extensible SDK (eSDK) <sdk-manual/sdk-manual>
-   Toaster Manual <toaster-manual/toaster-manual>
-   Test Environment Manual <test-manual/test-manual>
+   Overview and Concepts Manual <overview-manual/index>
+   Reference Manual <ref-manual/index>
+   Board Support Package (BSP) Developer's guide <bsp-guide/index>
+   Development Tasks Manual <dev-manual/index>
+   Linux Kernel Development Manual <kernel-dev/index>
+   Profile and Tracing Manual <profile-manual/index>
+   Application Development and the Extensible SDK (eSDK) <sdk-manual/index>
+   Toaster Manual <toaster-manual/index>
+   Test Environment Manual <test-manual/index>
    Bitbake User Manual <https://docs.yoctoproject.org/bitbake>
 
 .. toctree::