📝 site: fix BackTop been covered by footer

close #14093

site/theme/static/footer.less

@@ -7,7 +7,7 @@ footer {
   font-size: 14px;
   background-color: #000;
   position: relative;
-  z-index: 100;
+  z-index: 9;
   margin-left: -1px;
   color: rgba(255, 255, 255, 0.65);
 

site/theme/static/toc.less

@@ -52,8 +52,10 @@ ul.toc > li {
     background: #fff;
     overflow: auto;
     max-height: ~'calc(100vh - 16px)';
+    z-index: 9;
   }
 }
+
 .toc-affix-bottom {
   position: absolute;
   bottom: 88px;