Change markdown font weight to 600 (#13584)

Previously it was reduced globally from 700 to 500 but it actually needs
to be 600 to match GitHub's rendering.

Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: zeripath <art27@cantab.net>

web_src/less/_markdown.less

@@ -77,7 +77,7 @@
   h6 {
     margin-top: 1em;
     margin-bottom: 16px;
-    font-weight: 500;
+    font-weight: 600;
     line-height: 1.4;
 
     &:first-of-type {
@@ -193,7 +193,7 @@
     margin-top: 16px;
     font-size: 1em;
     font-style: italic;
-    font-weight: 500;
+    font-weight: 600;
   }
 
   dl dd {
@@ -224,7 +224,7 @@
   }
 
   table th {
-    font-weight: 500;
+    font-weight: 600;
   }
 
   table th,
@@ -450,7 +450,7 @@
   }
 
   .csv-data th {
-    font-weight: 500;
+    font-weight: 600;
     background: #f8f8f8;
     border-top: 0;
   }