Update the security guidance

[guardrex]

Fixes #35747

Tom, Wade ... Just need a single review on this one, too.

Originally, I planned to add a section on how to grab a Key Vault certificate's private and public keys for this, but there's just no time. I'm booked for several months between 10.0 and 'Blazorfy It!'™ issues. I don't even want to keep a backlog issue on this because I wouldn't reach it until at least 2026. However, I did make a tracking entry on #34434 to consider it next year.

After trying a few different approaches, I've settled on ...

• Use some AI-generated content that I like for the Establish public and private keys section.
• Use my original CAUTION note with modification in the Send a notification section.

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Internal previews

📄 File	🔗 Preview link
aspnetcore/blazor/progressive-web-app/push-notifications.md	aspnetcore/blazor/progressive-web-app/push-notifications

[Copilot]

Pull Request Overview

This PR updates the push notifications documentation to improve security guidance and standardize security warnings. The changes implement a consistent approach to handling private keys in ASP.NET Core applications by replacing custom security warnings with a standardized include file.

• Adds AI-usage metadata and updates the document date
• Replaces custom security warning text with a standardized include file reference
• Enhances security guidance with additional cryptographic best practices