fix: Simplify Gemfile to minimal configuration - only github-pages gem

Remove all version constraints and plugin specifications.
The github-pages gem includes all necessary Jekyll plugins.

Previous Gemfile had:
- Version constraints on github-pages (~> 227)
- Explicit plugin specifications (jekyll-feed, jekyll-sitemap, jekyll-seo-tag)
- Development gems (html-proofer, rake, webrick)
- Platform-specific gems (tzinfo, wdm)

These were causing gem resolution conflicts.

New minimal Gemfile:
- Only github-pages gem (latest stable)
- No version constraints
- No explicit plugins (included in github-pages)
- No development gems

This allows GitHub Actions to:
1. Use latest stable github-pages
2. Resolve all dependencies cleanly
3. Generate correct platform-specific Gemfile.lock
4. Build Jekyll site without conflicts

Expected result:
- Clean gem resolution
- Successful GitHub Pages build
- No dependency conflicts

Author: xaviercallens

Gemfile

@@ -1,27 +1,5 @@
 source "https://rubygems.org"
 
-# GitHub Pages gem - Stable version compatible with GitHub Pages
-gem "github-pages", "~> 227", group: :jekyll_plugins
-
-# Jekyll plugins (included in github-pages, but explicitly listed for clarity)
-group :jekyll_plugins do
-  gem "jekyll-feed"
-  gem "jekyll-sitemap"
-  gem "jekyll-seo-tag"
-end
-
-# Development and testing gems
-group :development, :test do
-  gem "html-proofer", "~> 3.19"
-  gem "rake", "~> 13.0"
-  gem "webrick", "~> 1.7"
-end
-
-# Windows and JRuby specific gems
-platforms :mingw, :x64_mingw, :mswin, :jruby do
-  gem "tzinfo", "~> 1.2"
-  gem "tzinfo-data"
-end
-
-# Performance-booster for watching directories on Windows
-gem "wdm", "~> 0.1.1", :platforms => [:mingw, :x64_mingw, :mswin]
+# GitHub Pages gem - Use latest stable version
+# This includes all necessary Jekyll plugins
+gem "github-pages", group: :jekyll_plugins