Windows: reorganize localized documentation, each in a dedicated folder. Adapt installers and documentation URL logic.

Now we have 3 folders under html directory：en, ru, zh-cn. Similar separation exist in online version:
https://veracrypt.fr/en
https://veracrypt.fr/ru
https://veracrypt.fr/zh-cn

doc/html/zh-cn/styles.css

@@ -0,0 +1,82 @@
+#menu {
+    display: none;
+}
+
+#menu ul {
+    list-style-type: none;
+    margin: 0px auto;
+    padding: 0;
+    overflow: hidden;
+    background-color: #08aad7;
+}
+
+#menu li {
+    float: left;
+}
+
+#menu li a {
+    display: block;
+    color: white;
+    text-align: center;
+    padding: 16px;
+    text-decoration: none;
+}
+
+#menu li a.active {
+    background-color: #84c7b0;
+}
+
+#menu li a:hover {
+    background-color: #111111;
+}
+
+#currency-form select:hover, #currency-form input[type="submit"]:hover {
+    background-color: #111111;
+    color: white;
+}
+
+#currency-form select {
+    border: 1px solid #08aad7;
+    border-radius: 3px;
+}
+
+#currency-form input[type="submit"] {
+    border-radius: 3px;
+}
+
+.donation-buttons {
+    display: flex;
+    flex-wrap: wrap;
+    gap: 15px;
+    margin: 20px 0;
+}
+
+.donation-button {
+    display: flex;
+    flex-direction: column;
+    align-items: center;
+    min-width: 100px;
+}
+
+.donation-button a {
+    display: block;
+    text-decoration: none;
+}
+
+.donation-button img {
+    display: block;
+    max-width: 100%;
+    height: auto;
+}
+
+.currency-label {
+    margin-top: 5px;
+    text-align: center;
+    font-size: 0.9em;
+}
+
+.donation-button a:hover {
+    opacity: 0.9;
+    transform: translateY(-1px);
+    transition: all 0.2s ease;
+}