feat: GitHub Pages deployment workflow and step support

[hoegertn]

Description

This PR adds comprehensive support for deploying documentation to GitHub Pages through both a standalone workflow and as a reusable pipeline step.

Changes

New Components:

• GithubPagesWorkflow: A new component that creates a complete GitHub Actions workflow for building and deploying documentation to GitHub Pages
• GithubPagesDeployStep: A reusable pipeline step that can be integrated into CDK pipelines or other workflows

Features:

• Configurable build commands and documentation folder paths
• Support for custom domains with automatic CNAME file generation
• Flexible Node.js version selection
• Optional dependency installation with custom install command support
• Custom runner tags for self-hosted runners
• Proper GitHub Actions permissions (contents:read, pages:write, id-token:write)
• Multi-platform support (GitHub, GitLab, Bash)
• Integration with existing CDK Pipeline infrastructure

Configuration Options:

• buildCommand: Required - command to generate documentation
• docsFolder: Output directory (default: 'docs')
• customDomain: Optional custom domain for GitHub Pages
• workflowName: Workflow file name (default: 'github-pages')
• branchName: Trigger branch (default: 'main')
• runnerTags: Runner configuration (default: ['ubuntu-latest'])
• nodeVersion: Node.js version (default: '20')
• runInstall: Enable/disable dependency installation (default: true)
• installCommand: Custom install command
• artifactName: GitHub Pages artifact name (default: 'github-pages')

Testing

Comprehensive test coverage includes:

• Default configuration behavior
• Custom options (domain, folder, branch, runner tags, Node version)
• Integration with CDK pipelines as post-deployment steps
• Multi-platform step configuration (GitHub, GitLab, Bash)
• Snapshot tests for generated workflow files

Example Usage

// Standalone workflow
new GithubPagesWorkflow(project, {
  buildCommand: 'npm run build:docs',
  docsFolder: 'dist/docs',
  customDomain: 'docs.example.com',
});

// As CDK pipeline post-deploy step
const ghPagesDeploy = new GithubPagesDeployStep(app, {
  buildCommand: 'npm run build:api-docs',
  docsFolder: 'api-docs',
  customDomain: 'api.example.com',
});

new GithubCDKPipeline(app, {
  stages: [{
    name: 'prod',
    postDeploySteps: [ghPagesDeploy],
  }],
});