[doc][doxygen] Rename usage doc to Understanding DuMux

doc/doxygen/DoxygenDumuxLayout.xml

@@ -18,7 +18,7 @@ SPDX-License-Identifier: GPL-3.0-or-later
     <!-- For this write @addtogroup <groupname> at the top of that markdown page -->
     <tab type="user" url="@ref external-libraries" visible="yes" title="External libraries" intro=""/>
     <tab type="user" url="@ref examples-and-tutorials" visible="yes" title="Examples and tutorials" intro=""/>
-    <tab type="usergroup" url="@ref usage-documentation" visible="yes" title="Usage documentation" >
+    <tab type="usergroup" url="@ref understanding-dumux" visible="yes" title="Understanding DuMux" >
       <tab type="user" url="@ref runtime-parameters" visible="yes" title="Runtime parameters" intro=""/>
       <tab type="user" url="@ref running-in-parallel" visible="yes" title="Running in parallel" intro=""/>
       <tab type="user" url="@ref input-and-output-formats" visible="yes" title="Input and output formats" intro=""/>

doc/doxygen/groups/usage.md → doc/doxygen/pages/understanding-dumux.md

-# Usage documentation
+# Understanding DuMux
 
 This part of the documentation is dedicated to provide explanations,
-tips & tricks on how to use DuMux programs and will introduce you to some
-of the conventions. In particular, this includes how to configure a DuMux
-program at runtime, input and output file formats (I/O), and how to run
-DuMux programs in parallel.
+tips & tricks on how to use DuMux and will introduce you to some
+of the conventions and concepts.
 
 * @ref runtime-parameters
 * @ref running-in-parallel