overview: Be consistent with the index file

Most of the other sections use `index.rst` as the index file. This does
the same of the overview section..

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>

index.rst

@@ -13,7 +13,7 @@ Welcome to |main_project_name| documentation!
 .. toctree::
    :maxdepth: 1
       
-   overview/README
+   overview/index
    meta-ohos/index
    readme/README
    contributing/README

overview/README.rst → overview/index.rst

@@ -10,6 +10,4 @@
 .. toctree::
    :maxdepth: 2
    
-   
    about-openharmony
-