docs: adding custom fields documentation step by step

[efortish]

Restore "Adding Custom Fields to Registration" How-To Guide

Context

This PR restores the documentation guide for adding custom fields to the registration page in the Authn MFE. This guide was previously removed from the repository, but there is no clear trace of when or why it was deleted from the codebase.

The original file is: https://github.com/openedx/frontend-app-authn/blob/63d16364fc2aa6538dd876bd3f6fd5612a20d605/docs/how_tos/adding_custom_fields_to_the_registration.rst#b-add-fields-that-do-not-exist-in-the-user-profile-model

Problem

The documentation for extending registration fields is currently missing from the repository, making it difficult for operators to:

• Understand how to add custom fields to the registration page
• Configure REGISTRATION_EXTRA_FIELDS properly
• Extend the UserProfile model with custom fields
• Integrate custom forms with the Authn MFE

This documentation is essential for organizations that need to collect additional user information during registration beyond the standard fields (email, username, password, name).

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Documentation Location

The restored documentation is located at:

docs/how_tos/adding_custom_fields_to_the_registration.rst

Key Topics Covered

1. Introduction to custom registration fields
2. Difference between required and optional fields
3. Step-by-step configuration for existing fields
4. Advanced guide for custom UserProfile field extensions
5. Backend and frontend integration requirements
6. Configuration examples for different deployment methods

Testing

• I followed up the instructions and everything is working as expected

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.57%. Comparing base (bee0afd) to head (2b28d9f).
⚠️ Report is 11 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1595   +/-   ##
=======================================
  Coverage   87.57%   87.57%           
=======================================
  Files         125      125           
  Lines        2309     2309           
  Branches      644      644           
=======================================
  Hits         2022     2022           
  Misses        278      278           
  Partials        9        9           

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[mphilbrick211]

Hi @openedx/2u-infinity! Could someone please review / merge this for us?

[julrusak]

hi @mphilbrick211 ! The Infinity team is no longer at 2U, the new contractor team has been onboarded but I think similarly to the new team that replaced Phoenix, this new Auth team will not be maintainers of this application.
I'm happy to update any spreadsheets or docs required, feel free to connect with me!