Add installation and configuration guides
Merchant/integrator-facing wiki, following the format of the
unified-plugin-core wiki (logo sidebar, page-per-topic, checklists).
- Home: requirements, the seven payment methods and their currency
constraints, card display modes, what the plugin offers
- Installation: the seven setup steps, webhooks, lost-notification
recovery, logging, checklist
- Configuration: connecting a Payplug account, LIVE/TEST, per-method
options, multi-channel rules, why a method may not appear at checkout
Corrects four things the README gets wrong or leaves stale: the refund
gateway list omitted payplug_scalapay and payplug_wero; the EUR base
currency requirement applies to the EUR-only methods and integrated
payment, not to the card gateway in redirected or Hosted Fields mode;
the live webhook path is Sylius's per-payment-method notify route, not
the deprecated /payplug/ipn; and the Monolog snippet targets the
payplug channel rather than [Payum].
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>