website: shell cleanup — dead config, alt text, stray tags, vestigial assets

Remove three dead theme _config keys (google_analytics, root_domain,
moderncpp_version — none referenced anywhere). Strip the unmatched trailing
</a> from both book footers. Add alt text to the hero cover and header logo
images. Remove the commented-out Code/Exercises/Answers nav block (Answers
was never built). Delete vestigial assets: lang/de.svg (no German edition)
and the unused check/down/feed/search PNGs.

Author: changkun

website/themes/moderncpp/_config.yml

@@ -1,4 +1 @@
 site_description: "Modern C++ Tutorial | C++11 to C++26 On the Fly | 现代 C++ 教程 | 高速上手 C++11 到 C++26"
-google_analytics: 
-root_domain: changkun.de/modern-cpp
-moderncpp_version: 2.0.0

website/themes/moderncpp/layout/index.ejs

@@ -16,7 +16,7 @@
   <div id="hero">
     <div class="inner">
       <div class="left">
-        <img class="hero-logo" src="<%- url_for("/modern-cpp/assets/cover-2nd-en.png") %>">
+        <img class="hero-logo" src="<%- url_for("/modern-cpp/assets/cover-2nd-en.png") %>" alt="Modern C++ Tutorial book cover">
       </div><div class="right">
         <h4>Changkun Ou</h4>
         <h1>
@@ -35,7 +35,7 @@
   <div id="hero">
     <div class="inner">
       <div class="left">
-        <img class="hero-logo" src="<%- url_for("/modern-cpp/assets/cover-2nd.png") %>">
+        <img class="hero-logo" src="<%- url_for("/modern-cpp/assets/cover-2nd.png") %>" alt="现代 C++ 教程封面">
       </div><div class="right">
         <h4>欧长坤 著</h4>
         <h1>

website/themes/moderncpp/layout/page.ejs

@@ -27,7 +27,7 @@
     <div class="footer">
         <p>
           <a href="https://changkun.de">Changkun Ou</a> &copy; 2016-<%- new Date().getFullYear() %>.
-          The book is licensed under <a rel="license" href="https://creativecommons.org/licenses/by-nc-nd/4.0/">Creative Commons Attribution-NonCommercial-NoDerivatives 4.0</a>, code is open sourced under the <a href="https://opensource.org/licenses/MIT" target="_blank">MIT</a> License. </a>
+          The book is licensed under <a rel="license" href="https://creativecommons.org/licenses/by-nc-nd/4.0/">Creative Commons Attribution-NonCommercial-NoDerivatives 4.0</a>, code is open sourced under the <a href="https://opensource.org/licenses/MIT" target="_blank">MIT</a> License.
         </p>
         <p>
           If you like the book, you could <a href="/modern-cpp/about/en/donate.html">donate the author</a>.
@@ -37,7 +37,7 @@
     <div class="footer">
         <p>
           <a href="https://changkun.de">欧长坤</a> &copy; 2016-<%- new Date().getFullYear() %> 版权所有，
-          采用<a rel="license" href="https://creativecommons.org/licenses/by-nc-nd/4.0/">知识共享署名-非商业性使用-禁止演绎 4.0 国际许可协议许可</a>，代码使用 <a href="https://opensource.org/licenses/MIT" target="_blank">MIT</a> 协议开源。</a>
+          采用<a rel="license" href="https://creativecommons.org/licenses/by-nc-nd/4.0/">知识共享署名-非商业性使用-禁止演绎 4.0 国际许可协议许可</a>，代码使用 <a href="https://opensource.org/licenses/MIT" target="_blank">MIT</a> 协议开源。
         </p>
         <p>
             如果你认为本书对你起到了帮助，可以<a href="/modern-cpp/about/donate.html">资助作者</a>。

website/themes/moderncpp/layout/partials/header.ejs

@@ -2,10 +2,10 @@
 <div id="header">
   <a id="logo" href="<%- url_for("/modern-cpp/") %>">
     <% if (isEn) { %>
-      <img src="<%- url_for("/modern-cpp/assets/cover-2nd-en-logo.png") %>">
+      <img src="<%- url_for("/modern-cpp/assets/cover-2nd-en-logo.png") %>" alt="Modern C++ Tutorial logo">
       <span>Modern C++ Tutorial: C++11 to C++26 On the Fly</span>
     <% } else {%>
-      <img src="<%- url_for("/modern-cpp/assets/cover-2nd-logo.png") %>">
+      <img src="<%- url_for("/modern-cpp/assets/cover-2nd-logo.png") %>" alt="现代 C++ 教程 logo">
       <span>现代 C++ 教程：高速上手 C++11 到 C++26</span>
     <% } %>
   </a>

website/themes/moderncpp/layout/partials/main_menu.ejs

@@ -3,10 +3,6 @@
   <ul class="nav-dropdown">
     <li><ul>
       <li><a class="nav-link" href="<%- url_for("/modern-cpp/zh-cn/00-preface/") %>" >正文</a></li>
-      <!-- TODO -->
-      <!-- <li><a class="nav-link" href="<%- url_for("/modern-cpp/code/1/") %>" >代码</a></li>
-      <li><a class="nav-link" href="<%- url_for("/modern-cpp/exercises/1/") %>" >习题</a></li>
-      <li><a class="nav-link" href="<%- url_for("/modern-cpp/answers/1/") %>" >答案</a></li> -->
     </ul></li>
   </ul>
 </li>

website/themes/moderncpp/layout/partials/main_menu_en.ejs

@@ -3,10 +3,6 @@
 <ul class="nav-dropdown">
     <li><ul>
     <li><a class="nav-link" href="<%- url_for("/modern-cpp/en-us/00-preface/") %>" >Book</a></li>
-    <!-- TODO -->
-    <!-- <li><a class="nav-link" href="<%- url_for("/modern-cpp/code/1/") %>" >Code</a></li>
-    <li><a class="nav-link" href="<%- url_for("/modern-cpp/exercises/1/") %>" >Exercise</a></li>
-    <li><a class="nav-link" href="<%- url_for("/modern-cpp/answers/1/") %>" >Answers</a></li> -->
     </ul></li>
 </ul>
 </li>