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_footer.less

@@ -38,7 +38,6 @@
     #dokuwiki__footer {
         .main-footer {
             position: relative;
-            z-index: 2;
             box-sizing: border-box;
             background-color: @ini_background_site;
         }