[DOCS] Improve documentation about the integration with proxies. (#669)

* [DOCS] Improve documentation about the integration with proxies.

This improvement resolves #384.

* Update index.md

docs/deployment/supported-proxies/index.md

@@ -1,12 +1,48 @@
 ---
 layout: default
-title: Supported Proxies
+title: Proxy Integration
 parent: Deployment
 nav_order: 4
 has_children: true
 ---
 
-# Supported Proxies
+# Integration with proxies
 
-**Authelia** works in collaboration with reverse proxies. Here you can find
-the documentation of the configuration required for every supported proxies.
+**Authelia** works in collaboration with reverse proxies. In the sub-pages you
+can find the documentation of the configuration required for every supported
+proxy.
+
+If you are not aware of the workflow of an authentication request, reading this
+[documentation](./home/architecture) first is highly recommended.
+
+
+## How Authelia integrates with proxies?
+
+Authelia takes authentication requests coming from the proxy and targeting the 
+`/api/verify` endpoint exposed by Authelia. Two pieces of information are required for
+Authelia to be able to authenticate the user request:
+
+* The session cookie or a `Proxy-Authorization` header (see [single factor authentication](./features/single-factor)).
+* The target URL of the user request (used primarily for [access control](./features/access-control)).
+
+The target URL can be provided using one of the following ways:
+
+* With `X-Original-URL` header containing the complete URL of the initial request.
+* With a combination of `X-Forwarded-Proto`, `X-Forwarded-Host` and `X-Forwarded-URI` headers.
+
+In the case of Traefik, those headers are automatically provided and therefore don't
+appear in the configuration examples.
+
+## Redirection to the login portal
+
+The endpoint `/api/verify` has different behaviors depending on whether
+the `rd` (for redirection) query parameter is provided.
+
+If redirection parameter is provided and contains the URL to the login portal
+served by Authelia, the request will either generate a 200 response
+if the request is authenticated or perform a redirection (302 response) to the
+login portal if not authenticated yet.
+
+If no redirection parameter is provided, the response code is either 200 or 401. The
+redirection must then be handled by the proxy when an error is detected
+(see [nginx](./deployment/supported-proxies/nginx) example).