gdaldem: add page (#8451)

pages/common/gdaldem.md

@@ -0,0 +1,16 @@
+# gdaldem
+
+> Tool to analyze and visualize digital elevation models (DEM).
+> More information: <https://gdal.org/programs/gdaldem>.
+
+- Compute the hillshade of a DEM:
+
+`gdaldem hillshade {{path/to/input.tif}} {{path/to/output.tif}}`
+
+- Compute the slope of a DEM:
+
+`gdaldem slope {{path/to/input.tif}} {{path/to/output.tif}}`
+
+- Compute the aspect of a DEM:
+
+`gdaldem aspect {{path/to/input.tif}} {{path/to/output.tif}}`