chore: Update report documentation (#13408)

New API Documentation

GET
/api/v2/accounts/{account_id}/reports/first_response_time_distribution
  - Returns first response time distribution grouped by channel type
- Shows conversation counts in time buckets: 0-1h, 1-4h, 4-8h, 8-24h,
24h+
  - Parameters: since, until (Unix timestamps)

  GET /api/v2/accounts/{account_id}/reports/inbox_label_matrix
  - Returns a matrix of conversation counts for inbox-label combinations
  - Parameters: since, until, inbox_ids[], label_ids[]

  Fixes

  - Removed unused business_hours boolean parameter from
  /api/v2/accounts/{account_id}/summary_reports/channel
- Updated ReDoc script from unstable @next to stable @2.1.5 version to
fix empty swagger page

swagger/definitions/index.yml

@@ -225,6 +225,10 @@ agent_conversation_metrics:
   $ref: './resource/reports/conversation/agent.yml'
 channel_summary:
   $ref: './resource/reports/channel_summary.yml'
+first_response_time_distribution:
+  $ref: './resource/reports/first_response_time_distribution.yml'
+inbox_label_matrix:
+  $ref: './resource/reports/inbox_label_matrix.yml'
 
 contact_detail:
   $ref: ./resource/contact_detail.yml