docs: concept for CAPTCHA in the PWA

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docs/README.md

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 - [Guide - Data Handling with Mappers](./guides/data-handling-with-mappers.md)
 - [Guide - Working with ESLint](./guides/eslint.md)
 - [Guide - Accessibility](./guides/accessibility.md)
+- [Concept - Captcha](./concepts/captcha.md)
 
 ### Customization
 

docs/concepts/captcha.md

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+
+# CAPTCHA in the PWA
+
+## Introduction
+
+The PWA supports reCAPTCHA V2 as well as reCAPTCHA V3.
+Which CAPTCHA version is used depends on which CAPTCHA service is created and running in the ICM.
+The pure CAPTCHA functionality is implemented as a PWA extension and can be found in the _extensions_ folder.
+There are two basic CAPTCHA components (one for each CAPTCHA version) and a lazy CAPTCHA component that decides which one to use.
+
+The PWA uses the [Angular Formly framework](https://formly.dev/docs/guide/getting-started) to process web forms, see also [The Formly Guide](../guides/formly.md).
+This framework allows you to automatically generate your forms using predefined form field types.
+A special type `ish-captcha-field` has been implemented to support the CAPTCHA functionality.
+You can define a field of this type in any _Formly_ form, so that this form is validated according to CAPTCHA.
+
+## High Level Overview
+
+The following class diagram shows the major classes of the CAPTCHA workflow that are relevant for most use cases:
+
+![Captcha Class Diagram](captcha-class-diagram.png)
+
+- _CAPTCHA Basic Components_ contains the recommended necessary UI components to integrate the Google reCAPTCHA service.
+- The lazy component ensures that the component for the corresponding version is reloaded asynchronously.
+- The CAPTCHA facade contains methods to retrieve the sitekey and the reCAPTCHA version used.
+- The PWA uses the Formly framework for web forms. A separate type `ish-captcha-field` is available for the reCAPTCHA functionality.
+- The three components (_registration-page.component.ts_, _request-reminder-form.component.ts_, _contact-form.component.ts_) contain Formly forms that contain a field of type CAPTCHA.
+- When these forms are submitted, the CAPTCHA token is set as a header parameter for the subsequent REST request. (When using CAPTCHA V3, the CAPTCHA action is also set as a header parameter).
+- The CAPTCHA validation takes place on the ICM side. If the score exceeds the threshold configured in the managed service, the request is rejected.
+
+## Integration of the CAPTCHA Components
+
+There is a special CAPTCHA field type that makes it very easy to protect any web form using the CAPTCHA functionality.
+The following example shows how to add a CAPTCHA field of type `ish-captcha-field` to any Formly form.
+
+The component's TypeScript file looks like this:
+
+```ts
+export class ExamplePageComponent implements OnInit {
+  ...
+  fields: FormlyFieldConfig[];
+  exampleFormGroup = new UntypedFormGroup({});
+  ...
+  ngOnInit() {
+    ...
+    this.fields = [
+        ...
+        {
+          type: 'ish-captcha-field',
+          props: {
+            topic: 'forgotPassword',
+          },
+        },
+      ];
+  }
+ ...
+}
+```
+
+The `ish-captcha-field` type is registered in the _src.app.shared.formly.types.**types.module.ts**_.
+Additionally, the module binds the corresponding field component to this type.
+The topic to be set corresponds to the CAPTCHA channel preferences available in ICM.
+If these preferences are disabled in ICM, CAPTCHA validation for this topic is also disabled in the PWA.
+
+The following table shows the mapping between existing PWA CAPTCHA topic names and the CAPTCHA channel preferences in the ICM:
+
+| **CaptchaTopic**                     | **ICM Settings**                        |
+| ------------------------------------ | --------------------------------------- |
+| contactUs                            | Contact Us                              |
+| emailShoppingCart                    | E-mail Shopping Cart                    |
+| forgotPassword                       | Forgot password                         |
+| redemptionOfGiftCardsAndCertificates | Redemption of Gift Cards & Certificates |
+| register                             | Registration                            |
+
+> :exclamation: The topic value is also appended to the header of the request triggered by submitting the web form. This is necessary to support the [actions concept of Google reCAPTCHA v3](https://developers.google.com/recaptcha/docs/v3#actions).
+
+## Basic Components
+
+As you can see in the high-level overview, there are components that represent the actual CAPTCHA functionality.
+Depending on the version used, these components implement either widgets provided by Google or the reCAPTCHA token functionality.
+The [ng-recaptcha](https://github.com/DethAriel/ng-recaptcha) library was used for the implementation.
+
+### CAPTCHA V2 (captcha-v2.component.ts)
+
+To add the widgets provided by Google, you need to import the **ReCAPTCHAModule** of the _ng-recaptcha_ library.
+Furthermore, _captcha site key_ must be set to initialize the widget.
+This site key is a required option on the reCAPTCHA HTML element.
+It is also necessary to store the token determined by the CAPTCHA event as a form control parameter.
+This allows the response to be validated and handled accordingly.
+In case of an error, an error message is displayed; in case of success, the entire form is processed further.
+
+### CAPTCHA V3 (captcha-v3.component.ts)
+
+To implement a callback function to handle the token, you need to import the **RecaptchaV3Module** of the _ng-recaptcha_ library.
+This component will trigger a callback every two minutes to retrieve a current reCAPTCHA token.
+This token is then appended to the request triggered by the submitted form.
+The ICM backend will then validate the token.
+
+## Further References
+
+- [Intershop Knowledge Base | Concept - ReCaptcha v3](https://support.intershop.com/kb/index.php/Display/29X281)
+- [Intershop Knowledge Base | Concept - ReCaptcha v2](https://support.intershop.com/kb/index.php/Display/2794B3)
+- [Getting Started | Formly](https://formly.dev/docs/guide/getting-started)
+- [GitHub - DethAriel/ng-recaptcha: Angular component for Google reCAPTCHA](https://github.com/DethAriel/ng-recaptcha)
+- [Developer's Guide | reCAPTCHA | Google for Developers](https://developers.google.com/recaptcha/intro)