Implement CLI mode and Markdown parsing

- Add CLI mode distinction (gocv vs gocv serve)
- Add goldmark dependency for Markdown parsing
- Create content reading logic from ./content directory
- Implement Markdown to HTML conversion
- Add sample content/index.md for testing
- Update .gitignore for build artifacts

Co-Authored-By: Claude (glm-5) <noreply@anthropic.com>

content/index.md

@@ -0,0 +1,13 @@
+# Welcome to Go-CV
+
+This is a sample markdown file for testing the Go-CV static site generator.
+
+## Features
+
+- Pure Go implementation
+- Markdown to HTML conversion
+- No external dependencies for PDF generation
+
+## About
+
+This project generates static HTML from markdown files in the `./content` directory.