feat(rooch): add new instructions for exporting states for rebuilding indexer

[popcnt1]

Summary

1. Added detailed steps in the README to guide users through setting up the genesis state.
2. Improved the export command to handle optional paths more gracefully.

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Name	Status	Preview	Comments	Updated (UTC)
rooch-portal	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Sep 10, 2024 2:02am
rooch-portal-v2.1	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Sep 10, 2024 2:02am

1 Skipped Deployment

| Name | Status | Preview | Comments | Updated (UTC) | | :--- | :----- | :------ | :------- | :------ | | **rooch** | ⬜️ Ignored ([Inspect](https://vercel.com/rooch/rooch/H9RBqwXQM7dtpTb5t2ELLksDbj9r)) | [Visit Preview](https://rooch-git-fork-popcnt1-docs-statedbgenesis-rooch.vercel.app) | | Sep 10, 2024 2:02am |