Docs updates for E/A#93532 Document Explain link on SmartScan failures

[MelvinBot]

Explanation of Change

Companion help-site update for Expensify/App#93532, which surfaces a discoverable inline Explain link on RECEIPTSCANFAILED system messages. Selecting Explain opens a thread where Concierge gives a field-aware explanation of why the scan failed.

The existing Troubleshoot SmartScan Issues article already documented the failure message and the retry flow, but not the new Explain affordance. This PR adds a task-based ## How to understand why a SmartScan failed section that documents the Explain link.

What changed

• docs/articles/new-expensify/reports-and-expenses/Troubleshoot-SmartScan-Issues.md — added a section explaining the inline Explain link, with steps to open it and what Concierge returns. Included a non-rendered screenshot placeholder.

Convention compliance

• Verified the UI label is exactly Explain against src/languages/en.ts (common.explain: 'Explain').
• Heading is task-based and starts with a question word, per HELP_AUTHORING_GUIDELINES.md Section 2.
• Reused the existing failure-message copy verbatim ("Receipt scanning failed — missing ... Enter details manually.").

@Beamanator, please review the files changed and confirm they reflect the current behavior. Then mark this PR Ready for review.