docs: add intro text to imageryintro.html (OSGeo#4536)

This PR adds an overview text to imageryintro.html including a link to addons.

(aside, addons URL fix in g.gui.gmodeler.html)

Co-authored-by: Nicklas Larsson <[email protected]>
Co-authored-by: Vaclav Petras <[email protected]>

gui/wxpython/gmodeler/g.gui.gmodeler.html

@@ -466,7 +466,7 @@ <h2>SEE ALSO</h2>
 
 <p>
 See also selected user models available from
-<a href="https://grass.osgeo.org/grass78/manuals/addons/">GRASS Addons repository</a>.
+<a href="https://grass.osgeo.org/grass8/manuals/addons/">GRASS Addons repository</a>.
 
 <p>
 See also

imagery/imageryintro.html

@@ -2,6 +2,21 @@
 <!-- meta page index: imagery -->
 <h3>Image processing in general</h3>
 
+GRASS GIS provides a powerful suite of tools for the processing and
+analysis of geospatial raster data, including satellite imagery and
+aerial photography. Its image processing capabilities encompass a broad
+range of preprocessing operations, such as data import, georeferencing,
+radiometric calibration, and atmospheric correction. It is particularly
+suited for handling Earth observation data, enabling the analysis of
+multispectral and temporal datasets. GRASS GIS supports advanced
+functionalities such as image classification, sensor fusion, and point
+cloud statistics. The calculation of vegetation indices further enables
+analyses of environmental, agricultural, and land cover dynamics. An
+extensive collection of
+<a href="https://grass.osgeo.org/grass8/manuals/addons/#i">addons</a>
+further enhances its image processing capabilities, particularly in the
+context of Earth observation and remote sensing applications.
+
 <b>Digital numbers and physical values (reflection/radiance-at-sensor):</b>
 <p>
 Satellite imagery is commonly stored in Digital Numbers (DN) for