Update PowerSystemsTestData org reference from NREL-Sienna to Sienna-Platform

[Copilot]

This repo migration update removes legacy organization naming by replacing remaining NREL-Sienna references with Sienna-Platform. It also aligns links with the new org location where referenced in this repository.

• Scope

  • Updated the only in-repo org URL reference in README.md.
  • No package/module names or dataset content were modified.

• Organization reference update

  • Replaced:
    • https://github.com/NREL-Sienna
  • With:
    • https://github.com/Sienna-Platform

• Link consistency checks

  • Verified no occurrences remained for:
    • NREL-Sienna
    • nrel-sienna.github.io
    • Slack invite links requiring workspace slug migration

- This repository contains several data files in modeling packages in [Sienna](https://github.com/NREL-Sienna).
+ This repository contains several data files in modeling packages in [Sienna](https://github.com/Sienna-Platform).

Original prompt

Summary

This repository was previously under the NREL-Sienna GitHub organization and has now been moved to Sienna-Platform. All references to the old organization name must be updated throughout the codebase.

Changes Required

1. Replace all occurrences of NREL-Sienna with Sienna-Platform in all files (READMEs, docs, CI/CD workflows, config files, source comments).
2. Replace all occurrences of nrel-sienna.github.io with sienna-platform.github.io in all links and URLs.
3. Update any Slack invite links to direct to the core-sienna Slack workspace (slug: core-sienna).
4. Do NOT change package names, module names, or unrelated content.

The following is the prior conversation context from the user's chat exploration (may be truncated):

Assistant: ````list type="task"
data:
- title: 'Update all occurrences of NREL-Sienna to Sienna-Platform'
repository: Sienna-Platform/InfrastructureSystems.jl
url: https://github.com/copilot/tasks/pull/PR_kwDODBZLyM7UGa69?session_id=7568b9bc-7466-4082-90ac-05f5294d1bd1
state: queued
- title: 'Replace NREL-Sienna references with Sienna-Platform and update Slack links'
repository: Sienna-Platform/StorageSystemsSimulations.jl
url: https://github.com/copilot/tasks/pull/PR_kwDOImSdLc7UGdd2?session_id=898f0898-f645-40a4-bc83-92e4ed419d2f
state: queued

Agents are running across all repositories in parallel. Here's a full summary of all tasks dispatched so far:

| Repository | Task Link |
|---|---|
| `PowerSimulations.jl` | [View](https://github.com/copilot/tasks/pull/PR_kwDOBoX4rs7UGW-X?session_id=ccfa2799-2274-408f-939e-256e41ff719f) |
| `PowerFlows.jl` | [View](https://github.com/copilot/tasks/pull/PR_kwDOG9-Hw87UGY1E?session_id=6568f54e-0a28-4f56-b017-353e8b212eb1) |
| `InfrastructureSystems.jl` | [View](https://github.com/copilot/tasks/pull/PR_kwDODBZLyM7UGa69?session_id=7568b9bc-7466-4082-90ac-05f5294d1bd1) |
| `PowerSystems.jl` | [View](https://github.com/copilot/tasks/pull/PR_kwDOBswbIM7UGc3e?session_id=5a8a3e20-f42e-4395-9ca1-0406bd31015a) |
| `StorageSystemsSimulations.jl` | [View](https://github.com/copilot/tasks/pull/PR_kwDOImSdLc7UGdd2?session_id=898f0898-f645-40a4-bc83-92e4ed419d2f) |

The remaining repositories will continue to be dispatched. Each PR will replace all `NREL-Sienna` references, update `nrel-sienna.github.io` URLs to `sienna-platform.github.io`, and update Slack invite links to the **`core-sienna`** workspace.

User: @Copilot Accepted Confirmation: Confirm agent session

Assistant: ````list type="task"
data:
    - title: 'Update all occurrences of NREL-Sienna to Sienna-Platform'
      repository: Sienna-Platform/InfrastructureSystems.jl
      url: https://github.com/copilot/tasks/pull/PR_kwDODBZLyM7UGa69?session_id=7568b9bc-7466-4082-90ac-05f5294d1bd1
      state: queued
    - title: 'Update NREL-Sienna references to Sienna-Platform'
      repository: Sienna-Platform/SiennaTemplate.jl
      url: https://github.com/copilot/tasks/pull/PR_kwDOIcvD0M7UGeKX?session_id=5abc1068-51b7-4834-8fed-77a6390d099e
      state: queued

Agents continue to roll out across all 35 repositories. Full task list so far:

Repository	Task
PowerSimulations.jl	View
PowerFlows.jl	View
InfrastructureSystems.jl	View
PowerSystems.jl	View
StorageSystemsSimulations.jl	View
SiennaTemplate.jl	View

User: @copilot Accepted Confirmation: Confirm agent session

Assistant: ````list type="task"
data:
- title: 'Update all occurrences of NREL-Sienna to Sienna-Platform'
repository: Sienna-Platform/InfrastructureSystems.jl
url: https://github.com/copilot/tasks/pull/PR_kwDODBZLyM7UGa69?session_id=7568b9bc-7466-4082-90ac-05f5294d1bd1
state: queued
- title: 'Update organization references from NREL-Sienna to Sienna-Platform'
repository: Sienna-Platform/PowerSimulationsDynamics.jl
url: https://github.com/copilot/tasks/pull/PR_kwDOELQnZM7UGgSP?session_id=98d834cd-ba69-4052-8e2a-6c3d880fdfef
state: queued

Here's the updated full list of all tasks dispatched so far:

| Repository | Task |
|---|---|
| `PowerSimulations.jl` | [View](https://github.com/copilot/tasks/pull/PR_kwDOBoX4rs7UGW-X?session_id=ccfa2799-2274-408f-939e-256e41ff719f) |
| `PowerFlows.jl` | [V...

</details>



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*This pull request was created from Copilot chat.*
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