feat: add GH page generation for end-users tutorials

[Copilot]

Adds Jekyll-based GitHub Pages documentation site for end-user tutorials, migrating content from Gitbook to be hosted directly in the repository.

Changes

• Jekyll site structure (docs/): Complete Jekyll setup with just-the-docs theme, including Gemfile, config, layouts, and navigation
• GitHub Actions workflow (.github/workflows/jekyll-gh-pages.yml): Automated build and deploy to GitHub Pages on push to main
• Documentation content:
  • Buyer-side tutorials (purchase and redeem rNFT)
  • Seller-side guides (selling on dApp, profile/storefront creation, product creation, token-gated commerce)
  • Widget documentation (commit button, redemption widget, metaverse toolkit)
  • WooCommerce integration guide
• Assets: Supporting images for documentation

Site will be available at https://bosonprotocol.github.io/interface once merged.

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