Documentation: add 10.2 book link

Author: lemora

content/documentation/_index.md

@@ -9,6 +9,7 @@ weight: 5
 
 * [Installation and Getting started Tutorial for dCache](https://github.com/dCache/dcache/blob/master/docs/TheBook/src/main/markdown/dcache-minimal-installation.md)
 * The Book - A general guide for administrators
+    * [The Book v10.2](https://www.dcache.org/manuals/Book-10.2)
     * [The Book v10.1](https://www.dcache.org/manuals/Book-10.1)
     * [The Book v10.0](https://www.dcache.org/manuals/Book-10.0)
     * [The Book v9.2](https://www.dcache.org/manuals/Book-9.2)