docs: update site responsive

Author: tangjinzhou

site/src/layouts/header/Logo.vue

@@ -49,7 +49,7 @@ export default defineComponent({
   }
 }
 
-@media only screen and (max-width: var(--mobile-max-width)) {
+@media only screen and (max-width: 767.99px) {
   #logo {
     padding-right: 0;
     padding-left: 0;

site/src/layouts/header/SearchBox.less

@@ -125,7 +125,7 @@
   }
 }
 
-@media only screen and (max-width: var(--mobile-max-width)) {
+@media only screen and (max-width: 767.99px) {
   #search-box {
     display: none;
   }

site/src/layouts/header/index.less

@@ -42,7 +42,7 @@
   }
 }
 
-@media only screen and (max-width: var(--mobile-max-width)) {
+@media only screen and (max-width: 767.99px) {
   #header {
     text-align: center;
   }

site/src/theme/static/responsive.less

@@ -9,7 +9,7 @@
   cursor: pointer;
 }
 
-@media only screen and (max-width: var(--screen-lg)) {
+@media only screen and (max-width: 992px) {
   .toc-affix {
     display: none;
   }