docs: link user register to metadata for custom fields (#8117)

# Which Problems Are Solved

an admin wants to know how to register a new user with some custom
fields

# How the Problems Are Solved

We already had examples about user metadata and onboard new users, but
the link between those and how to use it was missing.

# Additional Context

Replace this example with links to related issues, discussions, discord
threads, or other sources with more context.
Use the Closing #issue syntax for issues that are resolved with this PR.
- Closes #7951

---------

Co-authored-by: Livio Spring <[email protected]>

docs/docs/guides/integrate/login-ui/username-password.mdx

@@ -12,6 +12,13 @@ sidebar_label: Username and Password
 First, we create a new user with a username and password. In the example below we add a user with profile data, a verified email address, and a password.
 [Create User Documentation](/apis/resources/user_service/user-service-add-human-user)
 
+### Custom Fields
+
+If you have custom fields you like to add to your users that are not provided by ZITADEL, you can add them to the metadata.
+Metadata are key value pairs you can use for additional user data.
+These fields can also be included in the token of the user, so you have access to it all the time.
+Read more about the metadata [here](/docs/guides/manage/customize/user-metadata)
+
 ### Request
 
 ```bash

docs/docs/guides/integrate/onboarding/end-users.mdx

@@ -102,7 +102,7 @@ Please follow the configuration guides for the needed providers: [Let Users Logi
 
 import OrgLoginDescription from "./_org_login_description.mdx";
 
-<OrgLoginDescription name="OrgLodinDescription" />
+<OrgLoginDescription name="OrgLoginDescription" />
 
 ### Build your own registration form
 
@@ -117,8 +117,11 @@ We do have a guide series on how to build your own login ui, which also includes
 
 You can find all the guides here: [Build your own login UI](/docs/guides/integrate/login-ui)
 
-The create user request also allows you to add metadata (key, value) to the user.
+#### Custom fields
+
+The [create user request](/docs/apis/resources/user_service/user-service-add-human-user) also allows you to add [metadata](/docs/guides/manage/customize/user-metadata) (key, value) to the user.
 This gives you the possibility to collect additional data from your users during the registration process and store it directly to the user in ZITADEL.
+Those metadata can also directly be included in the [token](/docs/guides/manage/customize/user-metadata#use-tokens-to-get-user-metadata) of the user.
 We recommend storing business relevant data in the database of your application, and only authentication and authorization relevant data in ZITADEL to follow the separation of concern pattern.
 
 #### Registration with Organization External Identity Provider

docs/docs/guides/manage/customize/user-metadata.md

@@ -219,6 +219,11 @@ An example response for your search looks like this:
 }
 ```
 
+## Register user with custom metadata
+
+When you build your own registration UI you have the possibility to have custom fields and add them to the metadata of your user.
+Learn everything about how to build your own registration UI [here](/docs/guides/integrate/onboarding/end-users#build-your-own-registration-form).
+
 ## Manage user metadata through the management API
 
 The previous methods allowed you to retrieve metadata only for the `sub` in the access token.