ORC-1263: Doc: add decimal type to ORC Website

### What changes were proposed in this pull request?

Add decimal type to ORC Website.
At present, there is no decimal type description in https://orc.apache.org/docs/types.html.

### Why are the changes needed?

### How was this patch tested?

Closes #1236 from zhangbutao/ORC-1263.

Authored-by: zhangbutao <zhangbutao@cmss.chinamobile.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>
(cherry picked from commit 432a7aa)
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

site/_docs/types.md

@@ -27,6 +27,8 @@ ORC provides a rich set of scalar and compound types:
   * varchar
 * Binary blobs
   * binary
+* Decimal type
+  * decimal
 * Date/time
   * timestamp
   * timestamp with local time zone