Add a read-viewer screenshot to the Desktop GUI docs

Embed a screenshot of the genomic track viewer (codon tracks plus the read
pileup, with the ALT bases highlighted across supporting reads) on the Desktop
GUI page, in both English and Spanish. Captured from the BamViewer demo harness
on the bundled scenario fixture.

Author: Paururo

docs/assets/gui-track-viewer.png

@@ -28,6 +28,12 @@ Descarga la última versión para tu plataforma desde la
 
 ## Visor de pistas genómicas
 
+<div style="text-align: center;" markdown>
+![Visor de pistas genómicas de get_MNV: pistas de codones y pileup de lecturas de un MNV](assets/gui-track-viewer.png){ width="840" }
+</div>
+
+*El visor de pistas para un MNV (`GCT → TCA`, Ala10Ser): pistas de codones y el pileup de lecturas, con las bases ALT resaltadas en las 20 lecturas que dan soporte.*
+
 Al seleccionar una fila de variante se abre una vista de estilo IGV que alinea, columna a columna:
 
 - una **regla** que marca las posiciones de la variante y la ventana mostrada;

docs/gui.es.md

@@ -26,6 +26,12 @@ Download the latest build for your platform from the
 
 ## Genomic track viewer
 
+<div style="text-align: center;" markdown>
+![get_MNV genomic track viewer: codon tracks and the read pileup for an MNV](assets/gui-track-viewer.png){ width="840" }
+</div>
+
+*The track viewer for an MNV (`GCT → TCA`, Ala10Ser): codon tracks plus the read pileup, with the ALT bases highlighted across all 20 supporting reads.*
+
 Selecting a variant row opens an IGV-style view that lines up, column by column:
 
 - a **ruler** marking the variant positions and the displayed window;