add billigv2 documentation

docs/technical/backend/billig_v2.md

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+# Billig Integration v2
+
+In the words of a great visionary:
+```
+OMFG we are switching away from billig
+Use what you've learnt from billig and find some experienced
+pangs to help you out. Good luck!
+```
+
+As an experienced pang, I will try my best to conquer the task of defining the new Billig and what that entails for Samf4
+
+
+## Usage
+The new way we handle payment for events is through [Stripe](https://stripe.com/). They handle card data and payment; all we need to do is interact with their API.
+
+The current implementation uses a form to send a POST request with the necessary values. Stripe handles the payment and reroutes back to samfundet.no where the ticket is shown.
+
+The POST request is sent to three different URLs depending on the environment:
+```
+production: https://billettsalg.samfundet.no/pay
+staging: https://billettsalg-staging.samfundet.no/pay
+development*: http://localhost:4567/pay
+
+*local is not set up for samf4
+``` 
+
+The request contains:
+- Ticket group and amount (for example: member/not member)
+- Ticket type (member-card/email)
+- membercard or email
+
+The current implemenation is found [here](https://github.com/Samfundet/Samfundet/blob/master/app/views/events/buy.html.haml)
+
+
+Additionally, samfundet.no also handles showing the ticket after a successful purchase. Stripe reroutes to https://www.samfundet.no/arrangement/purchase_callback/:tickets where it gets the different ticket IDs and retrieves them from the database. The frontend allows the PDF to be downloaded.