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-src/config.toml

@@ -1,9 +1,7 @@
 [site]
 title = "cpp-httplib"
 version = "0.35.0"
-base_url = "https://yhirose.github.io/cpp-httplib"
-# Base path for URL generation. Use "/cpp-httplib" for GitHub Pages,
-# or "" for local development (python3 -m http.server).
+hostname = "https://yhirose.github.io"
 base_path = "/cpp-httplib"
 
 [[nav]]
@@ -17,9 +15,7 @@ url = "https://github.com/yhirose/cpp-httplib"
 icon_svg = '<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><path d="M9 19c-5 1.5-5-2.5-7-3m14 6v-3.87a3.37 3.37 0 0 0-.94-2.61c3.14-.35 6.44-1.54 6.44-7A5.44 5.44 0 0 0 20 4.77 5.07 5.07 0 0 0 19.91 1S18.73.65 16 2.48a13.38 13.38 0 0 0-7 0C6.27.65 5.09 1 5.09 1A5.07 5.07 0 0 0 5 4.77a5.44 5.44 0 0 0-1.5 3.78c0 5.42 3.3 6.61 6.44 7A3.37 3.37 0 0 0 9 18.13V22"/></svg>'
 
 [i18n]
-default_lang = "en"
 langs = ["en", "ja"]
 
 [highlight]
 theme = "base16-eighties.dark"
-theme_light = "base16-ocean.light"