feat: add `d/network_pool`

[tenthirtyam]

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Summary of Pull Request

• Adds network pool data source: data/vcf_network_pool.
• Add documentation for the network pool datasource.
• Adds example for the network pool datasource.
• Removed duplicate d/vcf_certificates example.

Type of Pull Request

• [ ] This is a bug fix.
• [x] This is an enhancement or feature.
• [ ] This is a code style/formatting update.
• [ ] This is a documentation update.
• [ ] This is a refactoring update.
• [ ] This is a chore update
• [ ] This is something else. Please describe:

Related to Existing Issues

Closes #199

Test and Documentation Coverage

For bug fixes or features:

• [x] Tests have been completed.
• [x] Documentation has been added/updated.

Running tool: /opt/homebrew/bin/go test -timeout 30s -run ^TestAccDataSourceVcfNetworkPool$ github.com/vmware/terraform-provider-vcf/internal/provider

ok      github.com/vmware/terraform-provider-vcf/internal/provider  0.440s

Breaking Changes?

• [ ] Yes, there are breaking changes.
• [x] No, there are no breaking changes.

[github-actions[bot]]

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