[doc] Update doxygen main page. Links to release.

This makes the default doxygen.opengeosys.org page for the
current development and the releas documentation is
located on own, special page.

Documentation/mainpage.dox.in

 /** \mainpage OpenGeoSys ${OGS_VERSION} source code documentation
  *
- * \section intro_sec Introduction
+ * \section source_documentation Source code documentation
  *
- * This page only describes the source code itself. See the following links for additional information:
+ * You will find the Doxygen documentation for the development branch at
+ * https://doxygen.opengeosys.org.
  *
- * - Latest Doxygen site for current master: https://doxygen-latest.opengeosys.org
+ * The documenation for the latest OGS release is
+ * https://doxygen.opengeosys.org/release.
+ *
+ * \section useful_links Useful links
+ * - OGS home page: https://opengeosys.org/
+ * - Selected benchmarks: https://opengeosys.org/docs/benchmarks
  * - Developer guide: https://opengeosys.org/docs/devguide
- * - Data Explorer (Gui) manual: https://gitlab.opengeosys.org/ogs/data_explorer_manual/-/jobs/artifacts/master/raw/ogsde-man.pdf?job=build
+ * - Data Explorer (GUI) manual: https://gitlab.opengeosys.org/ogs/data_explorer_manual/-/jobs/artifacts/master/raw/ogsde-man.pdf?job=build
  * - Discussion forum: https://discourse.opengeosys.org
  *
  *

README.md

@@ -19,7 +19,7 @@ waste deposition. OGS is comprised of the THMC-simulator (simply referred to as
 
 - Good starting point for users as well as for developers is the [documentation][documentation]
 - Check your code against our [styleguide](http://ufz.github.io/styleguide/cppguide.xml)
-- Have a look at the [source code documentation for the current release][docs] or [for the latest commit][docs-latest]
+- Have a look at the [source code documentation for the nightly build][docs-nightly] or [for the latest release][docs-release]
 
 ## License ##
 
@@ -28,6 +28,6 @@ OpenGeoSys is distributed under the Modified BSD License. See the
 
 [ogs]: https://www.opengeosys.org
 [documentation]: https://www.opengeosys.org/docs/
-[docs]: https://doxygen.opengeosys.org
-[docs-latest]: https://doxygen-latest.opengeosys.org
+[docs-nightly]: https://doxygen.opengeosys.org/
+[docs-release]: https://doxygen.opengeosys.org/release
 [license-source]: https://github.com/ufz/ogs/blob/master/LICENSE.txt