docs: add Testnet account funding guide and link from README

[eleven-smg]

What

Adds docs/testnet-funding.md, a focused guide to funding and activating Stellar Testnet accounts with the SDK, and links it from the README documentation list.

The guide covers:

• Why a newly created account must be funded before it exists on-chain
• Funding a Testnet account with Friendbot via fundTestnetAccount
• Confirming activation by reading the balance with getBalance
• A troubleshooting table for common unfunded-account errors (404 account not found, Friendbot 429 rate limiting, and network timeouts)
• Notes on disposable Testnet funds, periodic Testnet resets, and never reusing Testnet keys on Mainnet

Why

Getting Started mentions funding inline, but there was no dedicated, discoverable reference for account activation and the errors developers hit when an account has not been funded. This fills that gap and cross-links Getting Started, Network Error Handling, and Security Best Practices rather than duplicating them.

Notes

Documentation only - no source or dependency changes.

Closes #135