add translation link

docs/manual/en/_static/custom.js

@@ -0,0 +1,10 @@
+addEventListener("load", ev =>{
+	var url = location.href;
+	var el = document.getElementsByTagName('h1')[0];
+	var insert = document.createElement('p');
+	insert.style['text-indent'] = 0;
+	insert.innerHTML = url.indexOf('/en/')>=0 ?
+		`<a href="${url.replace('/en/','/zh-cn/')}">[中文]</a>` :
+		`<a href="${url.replace('/zh-cn/','/en/')}">[English]</a>`;
+	el.parentNode.insertBefore(insert, el.nextSibling);
+});

docs/manual/en/conf.py

@@ -28,6 +28,7 @@
 html_static_path = ['_static']
 
 html_css_files = ['custom.css']
+html_js_files = ['custom.js']
 
 html_theme_options = {
     'collapse_navigation': False

docs/manual/zh-cn/_static/custom.js

@@ -0,0 +1,10 @@
+addEventListener("load", ev =>{
+	var url = location.href;
+	var el = document.getElementsByTagName('h1')[0];
+	var insert = document.createElement('p');
+	insert.style['text-indent'] = 0;
+	insert.innerHTML = url.indexOf('/en/')>=0 ?
+		`<a href="${url.replace('/en/','/zh-cn/')}">[中文]</a>` :
+		`<a href="${url.replace('/zh-cn/','/en/')}">[English]</a>`;
+	el.parentNode.insertBefore(insert, el.nextSibling);
+});

docs/manual/zh-cn/conf.py

@@ -28,6 +28,7 @@
 html_static_path = ['_static']
 
 html_css_files = ['custom.css']
+html_js_files = ['custom.js']
 
 html_theme_options = {
     'collapse_navigation': False