move 00300-spacetime-json.md into the right docs folder (#4779)

# Description of Changes

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here. -->

moves the docs file for spacetime.json into the right folder so that the
website shows it.


# API and ABI breaking changes

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corresponding GitHub label. -->

no

# Expected complexity level and risk

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1

# Testing

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reviewers to do,
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no testing needed

Author: xDovos