Update manuals for admb-12.3.

Author: johnoel

docs/manuals/index.md

@@ -4,6 +4,14 @@ title: ADMB 11.5 Manuals
 ---
 <h2>Manuals</h2>
 
+<b>ADMB 12.3</b>
+
+_Download from github repository._
+
+* [ADMB 12.3 Manual](https://github.com/admb-project/admb/releases/download/admb-12.3/admb-12.3.pdf)
+* [ADMB Random Effects 12.3 Manual](https://github.com/admb-project/admb/releases/download/admb-12.3/admbre-12.3.pdf)
+* [Autodif 12.3 Manual](https://github.com/admb-project/admb/releases/download/admb-12.3/autodif-12.3.pdf)
+
 <b>ADMB 12.1</b>
 
 _Download from github repository._