docs: add Profile Fields guidance

docs/advanced-customization/profile-fields.md

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+---
+description: Create and organize custom fields on member profiles.
+---
+
+# Profile Fields
+
+Profile Fields let you collect and display additional information on member profiles. In the Admin area, open **Customization** and select **Profile Fields**.
+
+## Field groups
+
+Create groups to organize related fields, such as professional details or interests. Groups can contain multiple fields, and groups and their fields appear on a member's profile in the order shown in the Profile Fields admin page. Arrange them in the order that makes sense to members.
+
+## Field options
+
+When you add a field, configure its label, input type, group, and display area. You can also add an optional description, set a placeholder, choose whether the field is active, and choose whether it appears during onboarding.
+
+The available input types are:
+
+- **Text field**: A single-line response, such as a job title or website.
+- **Text area**: A multi-line response, such as a short introduction.
+
+The input type controls how members enter their response. It does not control where the response appears on their profile.
+
+## Display areas
+
+Choose where a completed field appears on the member's profile:
+
+- **Header**: Displays the field in the main profile header. A maximum of three fields can be placed in the header, so reserve this area for the most important information.
+- **Left sidebar**: Displays the field in the profile's left sidebar. Use this area for other useful profile details.
+
+## Onboarding
+
+Enable **Show in onboarding** to include the field in the profile form shown during member onboarding. This setting is separate from the display area: it controls when members are prompted to fill in the field, not where their response appears on the profile.