docs: add a testnet funding (friendbot) guide (closes #136)

[dchaudhari7177]

Closes #136.

Adds docs/testnet-funding.md, linked from the README in two places (the two-minute quickstart step, and the docs list).

Does not duplicate freighter-setup.md

That guide already walks through browser funding, so this one links to it rather than restating it, and covers what it doesn't:

• What Friendbot actually does — creates the account and grants 10,000 XLM. An address with no account doesn't exist, which is why "account not found" is the usual first error.
• Testnet resets wipe accounts and deployed contracts, so a previously funded account can legitimately vanish.
• Stellar CLI identities — stellar keys generate alice --network testnet --fund and stellar keys fund alice --network testnet, matching the commands docs/deploy.md already uses. Anything deploying or invoking from a terminal needs a funded CLI identity, which is separate from the browser wallet.
• Plain HTTP (curl https://friendbot.stellar.org/?addr=…) for scripts and CI, and that op_already_exists is a success, not a failure.
• Verifying a funded account, and a troubleshooting table.

The Tributary-specific section

The part I think earns the page: a split's recipients are ordinary accounts.

• Paying a split fails if a recipient was never funded — even when the payer is funded.
• For non-XLM tokens each recipient needs a trustline, which Friendbot does not create. This maps directly onto the trustlineWarningTitle / trustlineWarningItem messages the dashboard already shows.
• Nested split ids are not accounts and need no funding — only leaf G… addresses do.

Plus a loop to mint several funded test recipients at once.

Verification

Docs-only. Every relative link in the new file resolves (checked programmatically), including the freighter-setup.md#3-fund-your-account-friendbot anchor. No Rust or app code touched, so the cargo / npm run build gates are unaffected.

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