add Creative Commons license for docs

Author: fantix

docs/theme/layout.html

@@ -95,6 +95,25 @@
             <div id="main-content" role="main">
                 {% block body %} {% endblock %}
             </div>
+            <p style="text-align: center; font-size: 12px;">
+                <a rel="license" href="http://creativecommons.org/licenses/by-sa/4.0/">
+                    <img
+                        alt="{{ "知识共享许可协议" if language.startswith('zh') else "Creative Commons License" }}"
+                        style="border-width:0"
+                        src="https://i.creativecommons.org/l/by-sa/4.0/80x15.png"
+                    />
+                </a>
+                <br/>
+                {{
+                    "本作品采用"
+                    if language.startswith('zh') else
+                    "This work is licensed under a"
+                }}<a rel="license" href="http://creativecommons.org/licenses/by-sa/4.0/">{{
+                        "知识共享署名-相同方式共享 4.0 国际许可协议"
+                        if language.startswith('zh') else
+                        "Creative Commons Attribution-ShareAlike 4.0 International License"
+                }}</a>{{ "进行许可。" if language.startswith('zh') else "." }}
+            </p>
         </div>
 
         <div class="col hide-on-small-only m3 right-nav">