docs(newsletter): Add newsletter signup form

[Tobbe]

This PR adds the form above to the main https://cedarjs.com website so that new visitors to the site can easily sign up to receive news of new Cedar releases etc

[netlify]

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[greptile-apps]

Greptile Summary

• Added Netlify Forms-powered newsletter subscription form to homepage with email validation and proper styling
• Created dedicated thank you page with community links and next steps for new subscribers

Confidence Score: 4/5

• This PR is safe to merge with minimal risk.
• The implementation is straightforward with proper HTML form validation and Netlify Forms integration. One potential concern is ensuring the form is properly detected by Netlify during build, which depends on the data-netlify attribute being present in the built HTML.
• Verify that docs/src/pages/index.tsx newsletter form is detected by Netlify during deployment.

Important Files Changed

Filename	Overview
docs/src/pages/index.tsx	Added newsletter subscription form using Netlify Forms with proper styling and email validation.

Sequence Diagram

sequenceDiagram
    participant User
    participant Browser
    participant Index as "index.tsx"
    participant Netlify as "Netlify Forms"
    participant ThankYou as "newsletter-thank-you.tsx"
    
    User->>Browser: "Visit homepage"
    Browser->>Index: "Load page"
    Index->>Browser: "Render newsletter form"
    User->>Browser: "Enter email and submit"
    Browser->>Netlify: "POST /newsletter-thank-you"
    Netlify->>Netlify: "Process form submission"
    Netlify->>Browser: "Redirect to thank you page"
    Browser->>ThankYou: "Load thank you page"
    ThankYou->>Browser: "Render confirmation message"

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[greptile-apps]

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