Restructure API examples, add more

[ndeet]

The idea is outlined in issue #1371 where we structure examples for eCommerce use case and also on managing BTCPay Server by creating users and stores. All the examples for PHP, Node.js and cURL have the same structure while differing in details according to language, e.g. you can't really do a webhook validation endpoint with cURL.

• [x] cURL
• [x] PHP
• [x] NodeJS
• [x] overview
• [x] testing

Fixes #1371

[ndeet]

@pavlenex this is now ready

[pavlenex]

Great, a dev would need to review these to get merged though.