Step-up auth: Fix indentation and order of documentation section

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What does this MR do and why?

Improves the documentation structure by reorganizing sections and fixing heading hierarchy in the step-up authentication configuration. This follow-up change addresses documentation formatting issues:

• Moves custom documentation links section after provider examples
• Changes provider sections from h4 (####) to h3 (###) headings
• Creates logical flow: general setup → provider examples → config

The reorganization ensures readers first learn provider-specific implementation before learning about optional documentation links configuration.

This is a follow-up to !200996 (merged) .

🛠️ with ❤️ at Siemens

References

Follow-up to !200996 (merged)

Screenshots or screen recordings

Documentation structure changes only.

When visiting the documenation page http://gdk.test:1313/administration/auth/oidc/#configure-custom-documentation-links-for-step-up-authentication , then you will notice that the table of content has changed in the navigation sidebar (right side).

Before	After

How to set up and validate locally

1. Navigate to the documentation file at doc/administration/auth/oidc.md or http://gdk.test:1313/administration/auth/oidc/#configure-custom-documentation-links-for-step-up-authentication
2. Verify the step-up authentication sections are properly ordered:
   • General step-up auth configuration
   • Keycloak provider example (h3 heading)
   • Microsoft Entra ID provider example (h3 heading)
   • Custom documentation links configuration
3. Confirm heading hierarchy is consistent throughout

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

MR Checklist (@gerardo-navarro)

• Changelog entry added, if necessary (N/A - docs only)
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug (N/A - docs only)
• Tested in all supported browsers (N/A
• docs only)
• Conforms to the code review guidelines
• Conforms to the style guides
• Conforms to the javascript style guides (N/A - docs only)
• Conforms to the database guides (N/A - docs only)
• Conforms to the merge request performance guidelines (N/A - docs only)

Related to #526598