Move English documentation to folder en_US

doc/en_US/api-authentication.markdown

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+API Authentication
+==================
+
+API endpoint
+------------
+
+URL: `https://YOUR_SERVER/jsonrpc.php`
+
+
+Default method (HTTP Basic)
+---------------------------
+
+### Application credentials
+
+- Username: `jsonrpc`
+- Password: API token on the settings page
+
+### User credentials
+
+- Username: username
+- Password: user password or personal access token
+
+The API use the [HTTP Basic Authentication Scheme described in the RFC2617](http://www.ietf.org/rfc/rfc2617.txt).
+
+
+Custom HTTP header
+------------------
+
+You can use an alternative HTTP header for the authentication if your server have a very specific configuration.
+
+- The header name can be anything you want, by example `X-API-Auth`.
+- The header value is the `username:password` encoded in Base64.
+
+Configuration:
+
+1. Define your custom header in your `config.php`: `define('API_AUTHENTICATION_HEADER', 'X-API-Auth');`
+2. Encode the credentials in Base64, example with PHP `base64_encode('jsonrpc:19ffd9709d03ce50675c3a43d1c49c1ac207f4bc45f06c5b2701fbdf8929');`
+3. Test with curl:
+
+```bash
+curl \
+-H 'X-API-Auth: anNvbnJwYzoxOWZmZDk3MDlkMDNjZTUwNjc1YzNhNDNkMWM0OWMxYWMyMDdmNGJjNDVmMDZjNWIyNzAxZmJkZjg5Mjk=' \
+-d '{"jsonrpc": "2.0", "method": "getAllProjects", "id": 1}' \
+http://localhost/kanboard/jsonrpc.php
+```
+
+Authentication error
+--------------------
+
+If the credentials are wrong, you will receive a `401 Not Authorized` and the corresponding JSON response.
+
+
+Authorization error
+-------------------
+
+If the connected user is not allowed to access to the resource, you will receive a `403 Forbidden`.