Documentation Site on GitHub Pages (#2376)

* Add initial documentations

* Update documentation for Basic Client and add WebSocket section

* feat: add a static site generator with multi-language support

- Introduced a new Rust-based static site generator in the `docs-gen` directory.
- Implemented core functionality for building sites from markdown files, including:
  - Configuration loading from `config.toml`.
  - Markdown rendering with frontmatter support.
  - Navigation generation based on page structure.
  - Static file copying and output directory management.
- Added templates for base layout, pages, and portal.
- Created a CSS file for styling and a JavaScript file for interactive features like language selection and theme toggling.
- Updated documentation source with new configuration and example pages in English and Japanese.
- Added a `justfile` target for building the documentation site.

* Add language/theme toggle functionality

- Created a new Japanese tour index page at docs/ja/tour/index.html
- Implemented navigation links for various sections of the cpp-httplib tutorial
- Added a language selector to switch between English and Japanese
- Introduced theme toggle functionality to switch between light and dark modes
- Added mobile sidebar toggle for better navigation on smaller screens

docs-src/pages/ja/tour/index.md

@@ -0,0 +1,16 @@
+---
+title: "A Tour of cpp-httplib"
+order: 1
+---
+
+cpp-httplibの基本を、順番に学んでいくチュートリアルです。各章は前の章の内容を踏まえて進む構成なので、1章から順に読んでください。
+
+1. [Getting Started](01-getting-started) — httplib.h の入手とHello Worldサーバー
+2. [Basic Client](02-basic-client) — GET/POST・パスパラメーターのリクエスト送信
+3. [Basic Server](03-basic-server) — ルーティング、パスパラメーター、レスポンスの組み立て
+4. [Static File Server](04-static-file-server) — 静的ファイルの配信
+5. [TLS Setup](05-tls-setup) — OpenSSL / mbedTLS のセットアップ
+6. [HTTPS Client](06-https-client) — HTTPSサイトへのリクエスト
+7. [HTTPS Server](07-https-server) — HTTPSサーバーの構築
+8. [WebSocket](08-websocket) — WebSocket通信の基本
+9. [What's Next](09-whats-next) — さらなる機能の紹介