add 2.3.0 release doc (#8119)

Author: Le-Zheng

docs/readthedocs/source/doc/release.md

@@ -1,6 +1,6 @@
 # Release Download
 
-- **Release 2.3.0-SNAPSHOT nightly build**
+- **Release 2.4.0-SNAPSHOT nightly build**
 <table border="1"
 cellpadding="10"
 >
@@ -10,11 +10,30 @@ cellpadding="10"
     </tr>
     <tr>
        <td>Spark 2.4.6 </td>
-       <td><a href="https://oss.sonatype.org/content/repositories/snapshots/com/intel/analytics/bigdl/bigdl-assembly-spark_2.4.6/2.3.0-SNAPSHOT/">download</a></td>
+       <td><a href="https://oss.sonatype.org/content/repositories/snapshots/com/intel/analytics/bigdl/bigdl-assembly-spark_2.4.6/2.4.0-SNAPSHOT/">download</a></td>
     </tr>
     <tr>
        <td>Spark 3.1.3 </td>
-       <td><a href="https://oss.sonatype.org/content/repositories/snapshots/com/intel/analytics/bigdl/bigdl-assembly-spark_3.1.3/2.3.0-SNAPSHOT/">download</a></td> 
+       <td><a href="https://oss.sonatype.org/content/repositories/snapshots/com/intel/analytics/bigdl/bigdl-assembly-spark_3.1.3/2.4.0-SNAPSHOT/">download</a></td> 
+    </tr>
+</table>
+<br>
+
+- **Release 2.3.0**
+<table border="1"
+cellpadding="10"
+>
+    <tr>
+        <td></td>
+        <td>BigDL (All in One)</td>
+    </tr>
+    <tr>
+       <td>Spark 2.4.6 </td>
+       <td><a href="https://repo1.maven.org/maven2/com/intel/analytics/bigdl/bigdl-assembly-spark_2.4.6/2.3.0/">download</a></td>
+    </tr>
+    <tr>
+       <td>Spark 3.1.3 </td>
+       <td><a href="https://repo1.maven.org/maven2/com/intel/analytics/bigdl/bigdl-assembly-spark_3.1.3/2.3.0/">download</a></td>
     </tr>
 </table>
 <br>