Enhance documentation and configuration for static site generator

- Update README for clarity and quick start instructions
- Refine default config.toml with hostname and base path
- Adjust index.md files for consistent heading levels
- Simplify CSS for code block styling and remove unnecessary theme switching
- Refactor SiteConfig to derive full base URL from hostname and base path
- Update MarkdownRenderer to remove light theme handling

docs-gen/defaults/pages/en/index.md

@@ -3,7 +3,7 @@ title: Welcome
 order: 0
 ---
 
-# Welcome
+## Welcome
 
 This is the home page of your documentation site.
 

docs-gen/defaults/pages/ja/index.md

@@ -3,7 +3,7 @@ title: ようこそ
 order: 0
 ---
 
-# ようこそ
+## ようこそ
 
 ドキュメントサイトのトップページです。