fix: pop-up definition overlaps header

The pop-up of the terms overlapped the header. Previously, changes were made to @diplodoc/client to solve the problem, with ref: diplodoc-platform/client#51

The changes caused new bug and it was decided to fix the initial bug in another way by changing the z-index of the terms themselves.

src/scss/_term.scss

@@ -16,7 +16,7 @@
 
     &_dfn {
         position: absolute;
-        z-index: 1000;
+        z-index: 100;
 
         width: fit-content;
         max-width: 450px;