feat(tangle-dapp): Add `robots.txt` to optimzie SEO

[AtelyPham]

Summary of changes

Provide a detailed description of proposed changes.

• Add robots.txt to optimzie SEO

Proposed area of change

Put an x in the boxes that apply.

• [ ] apps/bridge-dapp
• [ ] apps/hubble-stats
• [ ] apps/stats-dapp
• [x] apps/tangle-dapp
• [ ] apps/testnet-leaderboard
• [ ] apps/faucet
• [ ] apps/zk-explorer
• [ ] libs/webb-ui-components

Reference issue to close (if applicable)

Specify any issues that can be closed from these changes (e.g. Closes #233).

• Related https://github.com/webb-tools/webb-dapp/issues/2405

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