index.rst: add Supported Toolchains section

Signed-off-by: Grzegorz Gwozdz <grzegorz.gwozdz@huawei.com>

index.rst

@@ -74,6 +74,12 @@ daily life.
    oniro/supported-technologies/openthread
    oniro/supported-technologies/containers
 
+.. toctree::
+   :caption: Supported Toolchains
+   :maxdepth: 2
+
+   oniro/toolchains
+
 .. toctree::
    :caption: Troubleshoot
    :maxdepth: 2