Adding Basic concepts to doxygen

doc/doxygen/DoxygenDumuxLayout.xml

@@ -19,6 +19,7 @@ SPDX-License-Identifier: GPL-3.0-or-later
     <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 understanding-dumux" visible="yes" title="Understanding DuMux" >
+      <tab type="user" url="@ref basic-concepts" visible="yes" title="Basic concepts" intro=""/>
       <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/pages/basic-concepts.md

+# Basic concepts

doc/doxygen/pages/understanding-dumux.md

@@ -4,6 +4,7 @@ This part of the documentation is dedicated to provide explanations,
 tips & tricks on how to use DuMux and will introduce you to some
 of the conventions and concepts.
 
+* @ref basic-concepts
 * @ref runtime-parameters
 * @ref running-in-parallel
 * @ref input-and-output-formats