Transform the Table 3 with kramdown style

[kentakayama]

Solves Martin Thomson's review comment

Section 12 has a wonderful table that is drawn using ascii art, which is then converted to an SVG. Please use a table.

I've transformed the Table 3 because the original one has header field at the first (left) column, but it is usual that it locates at the first (top) row.
Additionally, I've converted it to kramdown style table.

The Table 3 in html file will be

and one in txt file will be

         +===========+=============+==============+==============+
         | Number of | Number of   | Use Case     | Recommended? |
         | Long-Term | Content     |              |              |
         | Keys      | Encryption  |              |              |
         |           | Keys (CEKs) |              |              |
         +===========+=============+==============+==============+
         | Same key  | Single      | Legacy       | No, bad      |
         | for all   | CEK per     | Usage        | practice     |
         | devices   | payload     |              |              |
         |           | shared      |              |              |
         |           | with all    |              |              |
         |           | devies      |              |              |
         +-----------+-------------+--------------+--------------+
         | One key   | Single      | Efficient    | Yes          |
         | per       | CEK per     | Payload      |              |
         | device    | payload     | Distribution |              |
         |           | shared      |              |              |
         |           | with all    |              |              |
         |           | devies      |              |              |
         +-----------+-------------+--------------+--------------+
         | One Key   | One CEK     | Point-to-    | Yes          |
         | per       | per payload | Point        |              |
         | device    | encryption  | Payload      |              |
         |           | transaction | Distribution |              |
         |           | per device  |              |              |
         +-----------+-------------+--------------+--------------+

                                  Table 3