documentation: add doxygen style sheet and update DoxygenLayout

doc/DoxygenLayout.xml

-<!--
-    libeisdrt - A library to calculate EIS Drts
-    Copyright (C) 2023 Carl Klemm <carl@uvos.xyz>
-
-    This file is part of libeisdrt.
-
-    libeisdrt is free software: you can redistribute it and/or modify
-    it under the terms of the GNU Lesser General Public License as published by
-    the Free Software Foundation, either version 3 of the License, or
-    (at your option) any later version.
-
-    libeisdrt is distributed in the hope that it will be useful,
-    but WITHOUT ANY WARRANTY; without even the implied warranty of
-    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-    GNU Lesser General Public License for more details.
-
-    You should have received a copy of the GNU Lesser General Public License
-    along with libeisdrt.  If not, see <http://www.gnu.org/licenses/>.
--->
+<?xml version="1.0" encoding="UTF-8"?>
 <doxygenlayout version="1.0">
-  <!-- Generated by doxygen 1.9.3 -->
+  <!-- Generated by doxygen 1.9.8 -->
   <!-- Navigation index tabs for HTML output -->
   <navindex>
     <tab type="mainpage" visible="yes" title=""/>
     <tab type="pages" visible="yes" title="" intro=""/>
-    <tab type="modules" visible="yes" title="" intro=""/>
+    <tab type="topics" visible="yes" title="" intro=""/>
+    <tab type="modules" visible="yes" title="" intro="">
+      <tab type="modulelist" visible="yes" title="" intro=""/>
+      <tab type="modulemembers" visible="yes" title="" intro=""/>
+    </tab>
     <tab type="namespaces" visible="yes" title="">
       <tab type="namespacelist" visible="yes" title="" intro=""/>
       <tab type="namespacemembers" visible="yes" title="" intro=""/>
@@ -62,7 +48,7 @@
     <briefdescription visible="yes"/>
     <includes visible="$SHOW_HEADERFILE"/>
     <inheritancegraph visible="$CLASS_GRAPH"/>
-    <collaborationgraph visible="$COLLABORATION_GRAPH"/>
+    <collaborationgraph visible="yes"/>
     <memberdecl>
       <nestedclasses visible="yes" title=""/>
       <publictypes title=""/>
@@ -161,8 +147,8 @@
   <file>
     <briefdescription visible="yes"/>
     <includes visible="$SHOW_INCLUDE_FILES"/>
-    <includegraph visible="$INCLUDE_GRAPH"/>
-    <includedbygraph visible="$INCLUDED_BY_GRAPH"/>
+    <includegraph visible="yes"/>
+    <includedbygraph visible="yes"/>
     <sourcelink visible="yes"/>
     <memberdecl>
       <interfaces visible="yes" title=""/>
@@ -198,9 +184,10 @@
   <!-- Layout definition for a group page -->
   <group>
     <briefdescription visible="yes"/>
-    <groupgraph visible="$GROUP_GRAPHS"/>
+    <groupgraph visible="yes"/>
     <memberdecl>
       <nestedgroups visible="yes" title=""/>
+      <modules visible="yes" title=""/>
       <dirs visible="yes" title=""/>
       <files visible="yes" title=""/>
       <namespaces visible="yes" title=""/>
@@ -246,6 +233,25 @@
     <authorsection visible="yes"/>
   </group>
 
+  <!-- Layout definition for a C++20 module page -->
+  <module>
+    <briefdescription visible="yes"/>
+    <exportedmodules visible="yes"/>
+    <memberdecl>
+      <concepts visible="yes" title=""/>
+      <classes visible="yes" title=""/>
+      <enums title=""/>
+      <typedefs title=""/>
+      <functions title=""/>
+      <variables title=""/>
+      <membergroups title=""/>
+    </memberdecl>
+    <detaileddescription title=""/>
+    <memberdecl>
+      <files visible="yes"/>
+    </memberdecl>
+  </module>
+
   <!-- Layout definition for a directory page -->
   <directory>
     <briefdescription visible="yes"/>

doc/libeisdrt.doxygen.in

@@ -1201,7 +1201,7 @@ HTML_STYLESHEET        =
 # list). For an example see the documentation.
 # This tag requires that the tag GENERATE_HTML is set to YES.
 
-HTML_EXTRA_STYLESHEET  =
+HTML_EXTRA_STYLESHEET  = @CMAKE_CURRENT_SOURCE_DIR@/doc/style.css
 
 # The HTML_EXTRA_FILES tag can be used to specify one or more extra images or
 # other source files which should be copied to the HTML output directory. Note