[release] Use file extension for Doxygen only files.

Fixes FS#FS#1649.

doc/doxygen/Doxylocal

 # Where to search and which files to use
-INPUT                 += @srcdir@/mainpage \
-                         @srcdir@/modules \
-                         @top_srcdir@/dune/common/modules \
+INPUT                 += @srcdir@/mainpage.txt \
+                         @srcdir@/modules.txt \
+                         @top_srcdir@/dune/common/modules.txt \
                          @top_srcdir@/dune/common
 EXCLUDE               += @top_srcdir@/dune/common/test \
                          @top_srcdir@/dune/common/debugallocator.cc

dune/common/modules → dune/common/modules.txt

@@ -2,13 +2,13 @@
    documentation within the dune-common module. It works like this:
 
    @defgroup commands appear only in this file here which is
-   parsed before the other files (because it is mentioned first 
+   parsed before the other files (because it is mentioned first
    in the Doxyfile).
 
    Only @addtogroup is used in the code documentation.
 */
 
   /**
-	 @defgroup COGeometryType GeometryType
-	 @ingroup Common
+   * @defgroup COGeometryType GeometryType
+   * @ingroup Common
   */