Visualization doc: How to scale point size in ParaView (trixi-framework#779)

* revise doc

* fix typos

* add macro

* change suggestions

---------

Co-authored-by: LasNikas <niklas.nehe@web.de>

Author: LasNikas

docs/src/visualization.md

@@ -18,10 +18,65 @@ Follow these steps to view the exported VTK files in ParaView:
 You will now see the following:
 ![image](https://github.com/user-attachments/assets/383d323a-3020-4232-9dc3-682b0afe8653)
 
-It is useful to make the particles larger.
-For this, **first** make sure you have "fluid_1.pvd" highlighted in the "Pipeline Browser" then in the "Properties" window in the bottom left change "Point Size" to a larger value.
-![image](https://github.com/user-attachments/assets/6e975d2c-82ed-4d53-936b-bb0beafaf515)
+It is useful to make the dot size dependent on the actual particle size.
+For this, first make sure you have "fluid_1.pvd" highlighted in the "Pipeline Browser".
+Then, in the Properties panel (bottom left), adjust the following settings:
+1. "Representation" to "Point Gaussian".
+2. Choose the right "Shader Preset": "Plain Circle" for 2D and "Sphere" for 3D.
+3. Activate "Scale by Array" and select "`particle_spacing`" in "Gaussian Scale Array".
+4. Deactivate "Use Scale Function".
+5. Set the "Gaussian Radius" to "`0.5`".
+![image](https://github.com/user-attachments/assets/194d9a09-5937-4ee4-b229-07078afe3ff0)
 
+#### Visualization with Macro
+To simplify the visualization of your particle data in ParaView, you can use a macro.
+This macro automates the manual steps in the previous section to a single click of a button.
+Install the macro as follows.
+
+1. **Save the macro code** (see below) as a `.py` file, e.g. `PointGaussianMacro.py`.
+2. Open **ParaView** and go to the top menu:
+   **Macros** → **Import New Macro...** → Select the saved `.py` file.
+3. The macro will now appear in the **Macros** menu and can optionally be pinned to the **toolbar**.
+4. **Load your dataset** into ParaView.
+5. **Select the dataset** in the Pipeline Browser.
+6. Click on the macro name in the **Macros** menu (or toolbar, if pinned) to run it.
+7. The Point Gaussian representation with `particle_spacing` scaling will be applied automatically.
+
+
+---
+
+#### Macro Code
+
+```python
+# trace generated using paraview version 5.13.1
+#from paraview
+#paraview.compatibility.major = 5
+#paraview.compatibility.minor = 13
+
+from paraview.simple import *
+paraview.simple._DisableFirstRenderCameraReset()
+
+# get active source
+source = GetActiveSource()
+
+# get active view
+renderView1 = GetActiveViewOrCreate('RenderView')
+
+# get display properties
+sourceDisplay = GetDisplayProperties(source, view=renderView1)
+
+# change representation type
+sourceDisplay.SetRepresentationType('Point Gaussian')
+
+# modified display properties
+sourceDisplay.ShaderPreset = 'Plain circle' # for 2D, change to 'Sphere' for 3D
+sourceDisplay.ScaleByArray = 1
+sourceDisplay.SetScaleArray = ['POINTS', 'particle_spacing']
+sourceDisplay.UseScaleFunction = 0
+sourceDisplay.GaussianRadius = 0.5
+```
+
+#### Show results
 To now view the result variables **first** make sure you have "fluid_1.pvd" highlighted in the "Pipeline Browser" then select them in the variable selection combo box (see picture below).
 Let's, for example, pick "density". To now view the time progression of the result hit the "play button" (see picture below).
 ![image](https://github.com/user-attachments/assets/10dcf7eb-5808-4d4d-9db8-4beb25b5e51a)