Update documentation for deprecated contact_details field

[KushanSingh-Kalvium]

Change(s)

Change Type: Changed

Change Category: Documentation

Changelog Entry:
Update documentation regarding the deprecated contact_details field on
Person and Organisation.

A schema note was added explaining that contact_details has been removed
and that contacts should instead be defined through the generic contacts
mapping at the AIResource level.

The migration guide now documents how existing one-to-one
contact_details mappings are migrated to the many-to-many contact link
table.

The upload documentation has also been updated to remove references to
contact_details and show usage of the contacts field instead.

How to Test

1. Run mkdocs build to verify the documentation builds successfully.
2. Run mkdocs serve and open http://127.0.0.1:8000.
3. Confirm that:
   • The schema note appears in the REST API documentation.
   • The migration guide contains the "Contact Metadata Changes" section.
   • The upload documentation no longer references contact_details.

Checklist

• Documentation has been updated to reflect the changes.
• A self-review has been conducted.
• The PR title matches the changelog entry's one-line description.

Tests are not required because this change only affects documentation.

Related Issues

Closes #563