Move API token instructions to the open.xdmod.org site.

[aaronweeden]

Description

The instructions for generating API tokens now appear on https://open.xdmod.org/data-analytics-framework.html. This PR completes the move of them from the README in the xdmod-data repository.

Motivation and Context

The functionality being documented is more closely tied to the XDMoD portal than to the xdmod-data package.

Tests performed

I reviewed the changes on my GitHub branch and made sure the links work.

Type of change:

• Refactoring / documentation update (non-breaking change which does not change functionality)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix, feature, or removal that would cause existing functionality to change)
• Release preparation

Checklist:

• The milestone is set correctly on the pull request
• The appropriate labels have been added to the pull request
• Updates have been made to the xdmod-notebooks repository as necessary, and the notebooks all run successfully