Fix IO doc build index

[chensuyue]

Fix opea-project/GenAIInfra#1150 doc IO build

[eero-t]

Fix opea-project/GenAIInfra#1150 doc IO build

@chensuyue CI error message asks to update some toctree. However, there's no file with such name in that repository:

GenAIInfra$ find -iname '*toctree*'
GenAIInfra$

And looking at this fix, an update is needed to another repository (docs), to a differently named file (index.rst).

Then, looking at the "docs" repo, there's just a single index.rst in the deploy dir. I don't see anything e.g. for internal documentation / things which do not need to be added to any generated end-user documentation (that is disconnected from the files in the directory the doc tries to document).

=> at minimum, the CI test error message should be fixed to say something like:
<file> is missing from documentation 'index.rst' index file at: https://github.com/opea-project/docs

But proper fix would also be having such indexes in the same repository with the files they are referring to, so that trivial things like documentation updates don't need PRs to multiple repos.