Split the section about embed=true parameter of embedded Streamlit apps

[whitphx]

Successor to #688

The embed=true query parameter has a noteworthy side effect, so its section should be separated.

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TODO: While the updated content is correct for the upstream Streamlit, HF Spaces' patched Streamlit always disables scrolling even when being served without embed=true, so we have to investigate it before publishing this change.

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint.

[osanseviero]

Hi @whitphx! Should we close this PR?

[whitphx]

@osanseviero Sorry it has been stale for a long time. I fixed it and marked as ready.

[julien-c]

is this still in process @whitphx? or can it be closed?