Add starter template for content-js-sdk in Next.js

[szymonuryga]

Added Next.js Template with content-js-sdk Integration

This PR introduces a fully configured Next.js 15 template designed to work seamlessly with the @episerver/content-js-sdk package. The template is currently available on Vercel Templates, and a free course explaining how to build it step-by-step is available at: opti-masterclass.vercel.app.

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🚀 Template Features (from uryga-template branch)

• Next.js 15 with App Router
• Static Site Generation (SSG) for optimal performance
• On-Demand Revalidation via webhooks for real-time content updates
• Draft Mode for content previews
• SEO Optimized with full metadata support
• Multi-language Support with automatic language detection
• Tailwind CSS and shadcn/ui for responsive, modern UI
• TypeScript for full type safety and developer experience

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📦 Included Content: uryga-template.episerverdata

This template includes a uryga-template.episerverdata file located in the root of the project, which contains all initial content for Optimizely SaaS CMS.

To import it into your Optimizely instance:

1. Log in to your Optimizely SaaS CMS.
2. Important: Add the "Polish" language in your CMS settings before importing.
3. Navigate to: Admin > Tools > Import Data.
4. Upload the uryga-template.episerverdata file from the root folder.
5. Follow the import wizard to complete setup.

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🔐 Required Environment Variables

Make sure to set the following environment variables in your project:

OPTIMIZELY_GRAPH_URL=https://cg.optimizely.com/content/v2
OPTIMIZELY_GRAPH_SINGLE_KEY=<your-content-graph-api-key>

OPTIMIZELY_CMS_CLIENT_ID=<your-cms-client-id>
OPTIMIZELY_CMS_CLIENT_SECRET=<your-cms-client-secret>

OPTIMIZELY_REVALIDATE_SECRET=<your-custom-revalidate-secret>

OPTIMIZELY_START_PAGE_URL=/start-page
OPTIMIZELY_CMS_HOST=<your-saas-cms-instance-url>

Note: OPTIMIZELY_START_PAGE_URL must explicitly define the route to your Start Page. Optimizely CMS does not automatically route / to the Start Page when using hierarchical routing.