docs: add file associations, single instance, and MSIX guides

[leaanthony]

Summary

• Add guides/distribution/file-associations.mdx — covers FileAssociations option, ApplicationOpenedWithFile event, wails.json config, and per-platform behaviour
• Add guides/distribution/single-instance.mdx — covers SingleInstanceOptions, SecondInstanceData, encrypted communication, and forwarding events to the frontend
• Add guides/build/msix.mdx — covers MSIX CLI flags, signing, file associations in MSIX, package assets, Store submission, and CI/CD

Quality Checklist

• Code examples verified against source (single_instance.go, application_options.go, msix.go, flags/msix.go, examples/file-association/main.go)
• FileAssociation struct fields match build-assets.go exactly
• SingleInstanceOptions and SecondInstanceData fields match single_instance.go exactly
• MSIX CLI flags match flags/msix.go exactly
• Internal links verified against existing files
• No hedging language or filler phrases
• OS-specific steps use <Tabs syncKey="os">

Closes #16

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/16-create-distribution-build-guides

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[github-actions]

⚠️ Missing Changelog Update

Hi @leaanthony, please update v3/UNRELEASED_CHANGELOG.md with a description of your changes.

This helps us keep track of changes for the next release.

[cloudflare-workers-and-pages]

Deploying wails with    Cloudflare Pages

Latest commit:	ff803c7
Status:	🚫  Build failed.

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[github-actions]

⚠️ Missing Changelog Update

Hi @leaanthony, please update v3/UNRELEASED_CHANGELOG.md with a description of your changes.

This helps us keep track of changes for the next release.