Clean up z-indexes

There have been some z-indexes that were removed and then reintroduced
through strange merges. This is an attempt to remove all z-indexes that
are unnecessary and to clearly state the purpose of the remaining
z-indexes in a comment to each index.

SPR-981

css/area_content.less

@@ -10,14 +10,12 @@
 
         #dokuwiki__content {
             position: relative;
-            z-index: 10;
 
             &::before {
                 content: '';
                 position: absolute;
                 top: 0;
                 left: 0;
-                z-index: -1;
                 height: @page-header_height;
                 box-sizing: border-box;
                 width: 100%;