Update NU3043.md with OpenSSL instructions

[kartheekp-ms]

Added OpenSSL instructions for computing SHA-2 hashes.

Fixes: https://github.com/NuGet/Client.Engineering/issues/3344

I have tested the script in WSL and the commands work as expected.

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docs/reference/errors-and-warnings/NU3043.md

• Line 54, Column 1: [Warning: code-block-unclosed - See documentation] Unclosed code block. Code blocks must begin and end with triple backticks (```).

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[Copilot]

Pull Request Overview

This PR adds OpenSSL instructions for computing SHA-2 hashes of certificates to the NU3043 error documentation, providing cross-platform alternatives to the existing PowerShell script.

• Added OpenSSL command examples for Linux/macOS users
• Reorganized existing content with section headers for better clarity
• Provided instructions for both PEM/CRT and DER certificate formats

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[donnie-msft]

I like that you removed "customers" above, so should that be done here, too?

[kartheekp-ms]

Addressed your feedback in the latest commit.

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