Define typical merchant's user-flow when troubleshooting an invoice

[pavlenex]

Design sprint helped us define some problems that we'd like to address. The core of it seems to be troubleshooting process in case of a customer support ticker.

• [ ] Create TroubleshootingInvoice.md
• [ ] Define typical user-flow
• [ ] Define typical actions based on different invoice statuses
• [ ] Compare different invoice statuses and actions between payment processors
• [ ] Record a video showcasing problem troubleshooting in BTCPay Server

Related to #8