docs: add networks guide

[Democodes-flash]

Overview

Adds a complete networks guide at docs/guides/networks.md for developers new to Stellar.

What this PR does

• Explains the difference between testnet and mainnet, including the Horizon and Soroban endpoints for each.
• Shows how to fund a testnet account with the Stellar Friendbot — both via the Stellar Laboratory and from code.
• Documents switching networks with the network prop on StellarProvider (including dynamic switching).
• Includes a warning about never testing with real funds on mainnet.
• Links to Stellar Laboratory and Stellar Expert for exploring the chain.

Checks

• docs/guides/networks.md exists with complete, real content
• Explains testnet vs mainnet and includes Friendbot funding steps
• Documents switching networks via the network prop and includes the mainnet warning
• Links to Stellar Laboratory and Stellar Expert
• No placeholder text or TODO comments; testnet only

Closes #106

[drips-wave]

@Democodes-flash Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits