[doc][doxygen] Update versions in installation instructions.

doc/doxygen/pages/installation.md

@@ -77,9 +77,9 @@ To clone the Dune core modules, run:
 
 ### 2.2 Obtaining the Dumux source code
 
-To clone the latest 3.7 release version, run
+To clone the latest 3.8 release version, run
 
-    git clone -b releases/3.7 https://git.iws.uni-stuttgart.de/dumux-repositories/dumux.git
+    git clone -b releases/3.8 https://git.iws.uni-stuttgart.de/dumux-repositories/dumux.git
 
 The master branch (developer version) can be cloned with
 
@@ -174,6 +174,7 @@ Only the following Dune and DuMux versions are compatible:
 | DuMux version     | Dune version              |
 |-------------------|---------------------------|
 | master            | 2.9, master               |
+| 3.8               | 2.9
 | 3.7               | 2.9                       |
 | 3.5, 3.6, 3.7-git | 2.8, 2.9                  |
 | 3.3, 3.4          | 2.7, 2.8                  |