Add Microsoft Store submission guidance for apps with ECCNs

[Copilot]

The export restrictions documentation previously explained when an ECCN (Export Commodity Classification Number) is needed for cryptographic apps but provided no guidance on what developers should do after obtaining one. This left a significant gap in the documentation, as developers had no clear path forward for submitting apps with ECCNs to the Microsoft Store.

This PR adds a comprehensive new section "Microsoft Store submission process for apps with ECCNs" that addresses the missing guidance by providing:

What's Added

Step-by-step submission process: Clear instructions on how to provide ECCN information during the Partner Center submission process, including specific references to the submission options page where this information should be provided.

Compliance responsibility clarification: Explicitly separates what developers are responsible for (obtaining ECCNs, ongoing compliance, government reporting) versus what Microsoft handles (distribution reporting for the Store platform itself).

Certification process details: Explains how Microsoft's review team evaluates ECCN information during app certification and what developers can expect during this process.

Clear next steps: Provides actionable guidance for developers who have already obtained an ECCN and need to know how to proceed with Microsoft Store submission.

The new section integrates seamlessly with existing Partner Center submission documentation and maintains consistency with Microsoft's established processes for handling restricted capabilities and compliance requirements.

Impact

This change directly addresses the concerns raised in the issue about missing post-ECCN guidance while maintaining the existing document structure. Developers will now have a complete path from identifying the need for an ECCN through successful Microsoft Store submission.

Fixes #3068.

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[alvinashcraft]

@GrantMeStrength Can you take a quick look at the guidance Copilot has added here regarding Store apps that use cryptography and let me know what you think of it? Is there someone who works on the Store team who could verify if this is accurate (and worthwhile adding)?