website/docs: add authenticator_sms stage docs

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

website/docs/flow/stages/authenticator_sms/index.md

@@ -0,0 +1,37 @@
+---
+title: SMS authenticator setup stage
+---
+
+This stage configures an SMS-based authenticator using either Twilio, or a generic HTTP endpoint.
+
+## Twilio
+
+Navigate to https://console.twilio.com/, and log in to your existing account, or create a new one.
+
+In the sidebar, navigate to *Explore Products*, then *Messaging*, and *Services* below that.
+
+Click on *Create Messaging Service* to create a new set of API credentials.
+
+Give the service a Name, and select *Verify users* as a use-case.
+
+In the next step, add an address from your Sender Pool. Instructions on how to create numbers are not covered here, please check the Twilio documentation [here](https://www.twilio.com/docs).
+
+The other two steps can be skipped using the *Skip setup* button.
+
+Afterwards, copy the value of **Messaging Service SID**. This is the value for the *Twilio Account SID* field in authentik.
+
+Navigate back to the root of your Twilio console, and copy the Auth token. This is the value for the *Twilio Auth Token* field in authentik.
+
+## Generic
+
+For the generic provider, a POST request will be sent to the URL you have specified in the *External API URL* field. The request payload looks like this
+
+```json
+{
+    "From": "<value of the *From number* field>",
+    "To": "<the phone number of the user's device>",
+    "Body": "<the token that the user needs to authenticate>,
+}
+```
+
+Authentication can either be done as HTTP Basic, or via a Bearer Token. Any response with status 400 or above is counted as failed, and will prevent the user from proceeding.

website/docs/flow/stages/authenticator_validate/index.md

@@ -5,8 +5,9 @@ title: Authenticator Validation Stage
 This stage validates an already configured Authenticator Device. This device has to be configured using any of the other authenticator stages:
 
 - [Duo authenticator stage](../authenticator_duo/index.md)
-- [TOTP authenticator stage](../authenticator_totp/index.md)
+- [SMS authenticator stage](../authenticator_sms/index.md).
 - [Static authenticator stage](../authenticator_static/index.md).
+- [TOTP authenticator stage](../authenticator_totp/index.md)
 - [WebAuth authenticator stage](../authenticator_webauthn/index.md).
 
 You can select which type of device classes are allowed.

website/sidebars.js

@@ -115,6 +115,7 @@ module.exports = {
             label: "Stages",
             items: [
                 "flow/stages/authenticator_duo/index",
+                "flow/stages/authenticator_sms/index",
                 "flow/stages/authenticator_static/index",
                 "flow/stages/authenticator_totp/index",
                 "flow/stages/authenticator_validate/index",