API examples streamlining

[ndeet]

Add more consistent examples through curl, php, js

ecommerce use case (accept payments):

• create invoice
• register webhook?
• process webhook
• issue refund

more sophisticated, management examples

• create store
• create user
• create api key

[pavlenex]

@ndeet is there something more we need to do on this one?

[ndeet]

This is still on my todo, the other linked issue was about API docs, this is more about making existing examples the same across multiple languages like here https://docs.btcpayserver.org/Development/GreenFieldExample/