[Follow-up] Programmatic alternative for faucet

[cheny0]

Add this information to the relevant documents. Reference: #247

> [!TIP]
>
> **Programmatic alternative.** The faucet UI POSTs to `https://testnet.blockchain.logos.co/faucet-backend/<your-chosen-key>`. You can call that endpoint directly from a script or headless host:
>
> ```sh
> curl -X POST "https://testnet.blockchain.logos.co/faucet-backend/<your-chosen-key>"
> ```
>
> The response includes the transaction hash; the explorer URL it returns currently uses the `devnet.blockchain.logos.co` subdomain (see issue #244 for the testnet/devnet branding split).