doc: list WebPImageConverter in the file format tables.

doc/file-formats.dox

@@ -423,6 +423,16 @@ Derived from @ref Trade::AbstractImageConverter.
 <td class="m-text-center m-primary">bundled</td>
 <td class="m-text-center m-primary">public&nbsp;domain</td>
 </tr>
+<tr><td colspan="6"></td></tr>
+
+<tr>
+<th>WebP (`*.webp`)</th>
+<td>`WebPImageConverter`</td>
+<td>@relativeref{Trade,WebPImageConverter}</td>
+<td class="m-text-center m-dim">@ref Trade-WebPImageConverter-behavior "none"</td>
+<td class="m-text-center m-danger">external</td>
+<td class="m-text-center m-success">BSD 3-clause</td>
+</tr>
 </table>
 
 @endparblock