Add missing indexes and a3U documentation in readme for gke blueprints

[parulbajaj01]

Updated Readme file with the following changes:

1. Added missing entry in index for gke-managed-hyperdisk.yaml & gke-managed-parallelstore.yaml blueprints
2. Added documentation for gke-a3-ultragpu.yaml

Submission Checklist

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• Fork your PR branch from the Toolkit "develop" branch (not main)
• Test all changes with pre-commit in a local branch #
• Confirm that "make tests" passes all tests
• Add or modify unit tests to cover code changes
• Ensure that unit test coverage remains above 80%
• Update all applicable documentation
• Follow Cluster Toolkit Contribution guidelines #

[annuay-google]

How were these changes tested?

[parulbajaj01]

Rendered the markup on GitHub itself and the changes look good.

[annuay-google]

LGTM