docs: Add docs to show env overrides

[neelaundhia]

Solves:
[DOCS]: Missing section about backup.yaml override with ENVs

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Welcome @neelaundhia! It looks like this is your first PR to zilliztech/milvus-backup 🎉

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@neelaundhia Please associate the related issue to the body of your Pull Request. (eg. “issue: #”)