index.rst: include modbus documentation

ModBUS documentation is included from oniro.git index but that index is
explicitly excluded here. Include it again from the local index for
documentation to build correctly.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>

index.rst

@@ -73,6 +73,7 @@ daily life.
    meta-openharmony/supported-technologies/openharmony
    oniro/supported-technologies/openthread
    oniro/supported-technologies/containers
+   oniro/supported-technologies/modbus
 
 .. toctree::
    :caption: Supported Toolchains