Fix broken links in serving documentation

[yashsahare05]

Updates broken documentation links reported in #2255 to their current official destinations.

Changes include:

• Updating the TensorFlow data type documentation link.
• Updating outdated Kubernetes documentation links for kubectl, Deployments, Services, and LoadBalancers.
• Updating the retired Docker Toolbox link.
• Replacing the outdated MNIST beginners tutorial link with the current TensorFlow beginner quickstart.

The signature_defs.md links were intentionally left unchanged to avoid overlapping with #2088.

Testing

• Ran git diff --check.
• Verified the changes are limited to documentation files.

Refs #2255

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