Update docs: Adding Custom Fields to the Registration Page

[Apgomeznext]

PR: openedx/frontend-app-authn#1595 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).

Documentation Location
The restored documentation is located at:

docs/how_tos/adding_custom_fields_to_the_registration.rst

Key Topics Covered
Introduction to custom registration fields
Difference between required and optional fields
Step-by-step configuration for existing fields
Advanced guide for custom UserProfile field extensions Backend and frontend integration requirements
Configuration examples for different deployment methods Testing
I followed up the instructions and everything is working as expected

[openedx-webhooks]

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[efortish]

LGTM thank you so much @Apgomeznext

[sarina]

I'm having trouble following this doc. Please ping me when the docs build passes so I can review it more thoroughly as a rendered page. Thanks!

[sarina]

In RST, code blocks use two backticks. And paths should also be in code blocks.

Suggested change

	#. Go to `Site configurations` in admin: {{LMS}}/admin/site_configuration/siteconfiguration/
	#. Go to ``Site configurations`` in admin: ``{{LMS}}/admin/site_configuration/siteconfiguration/``

[Apgomeznext]

@sarina @efortish, this PR is causing a lot of trouble, and we need to work on the content, so I will close the PR for the moment and reopen it when we have everything ready to stop creating noise in the repository, if you agree.

[sarina]

@Apgomeznext this is useful content, I pushed a commit that should unbreak the build for a better review

[sarina]

OK, the build rendered but I'm going to push another commit shortly that will fix a lot of the visual rendering issues. I do have some technical questions that I can work with @efortish on

[efortish]

Hello @sarina

Sure, let me know how can we enhance the instructions and I'll work on that

[sarina]

Closing in favor of #1451