[Docs] update Binary Backups and Export

[Rajakavitha1]

remove references to miniogateway.
See the slack thread: https://dgraphlabs.slack.com/archives/C010A8PCW5B/p1690819475292549

Description: Please include the following in your PR description:

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3. If this PR corresponds to a JIRA issue, include "Fixes DOC-###" in the PR description.
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Note: Create and edit docs in the main branch when you can, so that we only cherry-pick out of main, not into main.
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