feat(docs): add newsletter signup and subscriber welcome page

[stephanj]

Add a Mailjet newsletter signup section to the landing page and a dedicated welcome page for confirmed subscribers.

• Embed the Mailjet form on the home page in a light card so it stays readable on both light and dark themes
• Load the Mailjet embed script client-side and call iFrameResize() directly, since the script's own window 'load' init has already fired by the time the React effect runs
• Add /newsletter-welcome (noindex) as the post-confirmation landing page with CTAs to docs, marketplace, blog, and GitHub

Pull Request

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Description

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring
• Performance improvement
• Test addition/improvement

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Testing

How has this been tested?

• Unit tests
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Checklist

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• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published

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Breaking Changes

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