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/static/js/main.js

@@ -0,0 +1,73 @@
+// Language selector
+(function () {
+  var btn = document.querySelector('.lang-btn');
+  var popup = document.querySelector('.lang-popup');
+  if (!btn || !popup) return;
+
+  btn.addEventListener('click', function (e) {
+    e.stopPropagation();
+    popup.classList.toggle('open');
+  });
+
+  document.addEventListener('click', function () {
+    popup.classList.remove('open');
+  });
+
+  popup.addEventListener('click', function (e) {
+    var link = e.target.closest('[data-lang]');
+    if (!link) return;
+    e.preventDefault();
+    var lang = link.getAttribute('data-lang');
+    localStorage.setItem('preferred-lang', lang);
+    var path = window.location.pathname;
+    var newPath = path.replace(/^\/[a-z]{2}\//, '/' + lang + '/');
+    window.location.href = newPath;
+  });
+})();
+
+// Theme toggle
+(function () {
+  var btn = document.querySelector('.theme-toggle');
+  if (!btn) return;
+
+  function getTheme() {
+    var stored = localStorage.getItem('preferred-theme');
+    if (stored) return stored;
+    return window.matchMedia('(prefers-color-scheme: light)').matches ? 'light' : 'dark';
+  }
+
+  function applyTheme(theme) {
+    if (theme === 'light') {
+      document.documentElement.setAttribute('data-theme', 'light');
+    } else {
+      document.documentElement.removeAttribute('data-theme');
+    }
+    btn.textContent = theme === 'light' ? '\u2600\uFE0F' : '\uD83C\uDF19';
+  }
+
+  applyTheme(getTheme());
+
+  btn.addEventListener('click', function () {
+    var current = getTheme();
+    var next = current === 'dark' ? 'light' : 'dark';
+    localStorage.setItem('preferred-theme', next);
+    applyTheme(next);
+  });
+})();
+
+// Mobile sidebar toggle
+(function () {
+  var toggle = document.querySelector('.sidebar-toggle');
+  var sidebar = document.querySelector('.sidebar');
+  if (!toggle || !sidebar) return;
+
+  toggle.addEventListener('click', function () {
+    sidebar.classList.toggle('open');
+  });
+
+  document.addEventListener('click', function (e) {
+    if (!sidebar.contains(e.target) && e.target !== toggle) {
+      sidebar.classList.remove('open');
+    }
+  });
+})();