Add grid with links to user guide sections (#412)

Co-authored-by: stelliom <67868694+stelliom@users.noreply.github.com>

Author: mjaehn

docs/user_manual/index.md

@@ -2,10 +2,29 @@
 
 *Hermann Asensio / Martina Messmer / Daniel Lüthi / Katie Osterried / Jonas Jucker / Jacopo Canton / Philipp Sommer / Jürgen Helmert / Michael Jähn*
 
-This documentation provides an overall description of the **EXTPAR** software,
-provides detailed information on specific modules, current limitations
-and the namelist input.
+<div class="grid cards" markdown>
 
-For the sake of completeness, the latest EXTPAR documentation (v5.14) 
-that was created from the original LaTeX file, can be downloaded
-[here](EXTPAR_User_Manual_5_14.pdf).
+- :fontawesome-solid-book: [**Overall Description**](user_manual_01_overall_description.md)  
+  *Provides a general overview of the EXTPAR system and its purpose.*
+
+- :fontawesome-solid-gear: [**Software Modules**](user_manual_02_software_modules.md)  
+  *Details the software components and their interactions.*
+
+- :fontawesome-solid-code: [**Fortran Modules**](user_manual_03_fortran_modules.md)  
+  *Describes the Fortran modules used in the EXTPAR system.*
+
+- :fontawesome-brands-python: [**Python Modules**](user_manual_04_python_modules.md)  
+  *Explains the Python modules and their functionality.*
+
+- :fontawesome-solid-circle-exclamation: [**Current Limitations**](user_manual_05_current_limitations.md)  
+  *Highlights the known limitations and constraints of the system.*
+
+- :fontawesome-solid-list: [**Namelist Input**](user_manual_06_namelist_input.md)  
+  *Covers the structure and usage of namelist input files.*
+
+</div>
+
+!!! note
+    For the sake of completeness, the latest EXTPAR documentation (v5.14) 
+    that was created from the original LaTeX file, can be downloaded
+    [here](EXTPAR_User_Manual_5_14.pdf).