docs: fix documentation structure

[dandhlee]

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Updating the doc structure on cloud.google.com to match other libraries - removing duplicate entry pages like https://cloud.google.com/python/docs/reference/spanner/latest/streamed-api. Googlers see b/285104763 for more info.

This does not affect the overall structure of the documentation, merely taking out some unnecessary documents and updating wordings for better documentation experience.

Fixes b/285104763 🦕