docs(erp-toolkit): expand use cases with the Wilken integration set

Adds the use cases that production integrations (e.g. Wilken) ship today
but were missing from the docs.

Inbound:
- Sync Installment Schedule (Abschlagsplan as scheduled debit billing_events)
- Sync Portal User (PROFILE contact + portal_user provisioning)
- Enrich Sync Billing Events with running balance + debit/credit direction
- Clarify Keep Billing Account In Sync as three sub-domain mappings
  (account, billing address, payment method)
- Document File Proxy as the standard custom_download_url path for documents

Outbound:
- Submit Service Meter Reading (ServiceMeterReadingAdded, ticket-driven)
- Submit New Order (OrderSubmission, journey acquisition)
- Switch Tariff (TariffChange, product switch on existing contract)
- Remove Billing Account Connection (BillingAccountConnectionRemoved)
- Submit General Request (GeneralRequestCreated, contact form ticket)
- Replace Request Data Sync custom webhook with the real
  OnDemandSyncCustomerRequested / OnDemandSyncContractRequested events
- Realign existing payload-field lists with the actual context block
  (identityId / customerAccountId / contractNumber)
- Note that PaymentMethodUpdated webhooks carry context only; the middle
  layer must re-fetch the billing account to read the new payment method

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Co-authored-by: Claude <noreply@anthropic.com>

Author: anttiviljami