[Docs] Dgraph CLI Reference

[Rajakavitha1]

Remove references to raft migrate in the CLI.
Fixes: #600

Description: Please include the following in your PR description:

1. A brief, clear description of the change.
2. Link to any related PRs or discuss.dgraph.io posts.
3. If this PR corresponds to a JIRA issue, include "Fixes DOC-###" in the PR description.
4. If this is a breaking change, please put "[Breaking]" in the title. In the description, please put a note with exactly who these changes are breaking for.
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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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