Fix: Add Netlify build configuration for mermaid diagrams

[anthonymonori]

Problem

After merging #47, Netlify builds fail with:
```
Could not find gem 'jekyll-mermaid-prebuild (~> 0.3.0)' in locally installed gems.
```

Solution

This PR adds the necessary configuration for Netlify to:

1. Install Node.js 24
2. Install mermaid-cli (which requires Puppeteer/Chrome)
3. Install the chromium plugin for Puppeteer support
4. Use the updated Gemfile.lock with jekyll-mermaid-prebuild

Changes

• `package.json` - Declares `@mermaid-js/mermaid-cli` as dependency with prebuild script to install Chrome
• `.node-version` - Specifies Node.js 24 for Netlify
• `netlify.toml` - Configures build command and chromium plugin
• `Gemfile.lock` - Updated with jekyll-mermaid-prebuild gem

How it works

1. Netlify detects `.node-version` and installs Node.js 24
2. Netlify detects `package.json` and runs `npm install`
3. The `prebuild` script installs Chrome for Puppeteer
4. The `netlify-plugin-chromium` plugin ensures Chromium is available
5. Jekyll build runs with jekyll-mermaid-prebuild, rendering diagrams to SVG

Testing

After merging, the next Netlify build should succeed and render mermaid diagrams to SVG.