Add a troubleshooting guide for wallet and network mismatches

[ChaoLing140]

Backlog ID: #325

• Context: Contributors and users commonly hit network, RPC, and wallet configuration mismatches during setup.
• Acceptance criteria:
  • Common mismatch symptoms are listed with fixes.
  • The guide distinguishes testnet, mainnet, Horizon, and Soroban RPC settings.
  • The document points to the correct environment variables and files.
• Suggested tests:
  • Manual verification against the current .env.example files.
  • Link check for referenced setup docs.