Docs: Add a note of Omnibus baseline storage req

What does this MR do?

Under the storage section of the hardware requirements doc, this warns of how much space the Omnibus package takes up itself.

This addition was motivated by a customer ticket (internal) where they did not have enough space on the filesystem for the package. I felt that having it mentioned somewhere would be valuable, especially in cases like this, where we could provide a link to help them understand what is needed.

Related issues

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