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/components/panels.html

@@ -150,19 +150,19 @@
         </thead>
         <tbody>
           <tr>
-            <td>1</td>
+            <th scope="row">1</th>
             <td>Mark</td>
             <td>Otto</td>
             <td>@mdo</td>
           </tr>
           <tr>
-            <td>2</td>
+            <th scope="row">2</th>
             <td>Jacob</td>
             <td>Thornton</td>
             <td>@fat</td>
           </tr>
           <tr>
-            <td>3</td>
+            <th scope="row">3</th>
             <td>Larry</td>
             <td>the Bird</td>
             <td>@twitter</td>
@@ -204,19 +204,19 @@
         </thead>
         <tbody>
           <tr>
-            <td>1</td>
+            <th scope="row">1</th>
             <td>Mark</td>
             <td>Otto</td>
             <td>@mdo</td>
           </tr>
           <tr>
-            <td>2</td>
+            <th scope="row">2</th>
             <td>Jacob</td>
             <td>Thornton</td>
             <td>@fat</td>
           </tr>
           <tr>
-            <td>3</td>
+            <th scope="row">3</th>
             <td>Larry</td>
             <td>the Bird</td>
             <td>@twitter</td>