index.rst: add Supported Toolchains section

Right now pipelines fail for some MRs, because toolchain.rst is not added to index.rst

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