Fixes to scope

as discssued in
https://github.com/twbs/bootstrap/issues/15117#issuecomment-63029993

- added ``scope="row"`` to row headers
- made sure row headers are actual ``<th>`` elements
- removed ``scope="col"`` where it was redundant
- simplified table examples with (to me) unnecessary
``rowspan``/``colspan`` (unless you really want complex tables, in which
case we would need full-on ``id`` and ``headers`` attributes to make
sure each table cell has an explicit association with the related header
cells - overkill, in my opinion)
- one table (in typography) left header-less, as it's more or less presentational (though its structure is still important, hence not adding ``role="presentation"``)

docs/_includes/css/sass.html

@@ -15,27 +15,27 @@
       </thead>
       <tbody>
         <tr>
-          <th><code>lib/</code></th>
+          <th scope="row"><code>lib/</code></th>
           <td>Ruby gem code (Sass configuration, Rails and Compass integrations)</td>
         </tr>
         <tr>
-          <th><code>tasks/</code></th>
+          <th scope="row"><code>tasks/</code></th>
           <td>Converter scripts (turning upstream Less to Sass)</td>
         </tr>
         <tr>
-          <th><code>test/</code></th>
+          <th scope="row"><code>test/</code></th>
           <td>Compilation tests</td>
         </tr>
         <tr>
-          <th><code>templates/</code></th>
+          <th scope="row"><code>templates/</code></th>
           <td>Compass package manifest</td>
         </tr>
         <tr>
-          <th><code>vendor/assets/</code></th>
+          <th scope="row"><code>vendor/assets/</code></th>
           <td>Sass, JavaScript, and font files</td>
         </tr>
         <tr>
-          <th><code>Rakefile</code></th>
+          <th scope="row"><code>Rakefile</code></th>
           <td>Internal tasks, such as rake and convert</td>
         </tr>
       </tbody>