docs: update regarding the removal of richdem from the source code

CONTRIBUTING.md

@@ -50,6 +50,8 @@ pytest
 
 Running `pytest` will trigger a script that automatically downloads test data from [https://github.com/GlacioHack/xdem-data](https://github.com/GlacioHack/xdem-data) used to run all tests.
 
+RichDEM should only be used for testing purposes within the xDEM project. The functionality of xDEM must not depend on RichDEM.
+
 ### Documentation
 
 If your changes need to be reflected in the documentation, update the related pages located in `doc/source/`. The documentation is written in MyST markdown syntax, similar to GitHub's default Markdown (see [MyST-NB](https://myst-nb.readthedocs.io/en/latest/authoring/text-notebooks.html) for details).

README.md

@@ -30,6 +30,9 @@ relies on [Rasterio](https://github.com/rasterio/rasterio), [GeoPandas](https://
 for georeferenced calculations, and on [NumPy](https://github.com/numpy/numpy) and [Xarray](https://github.com/pydata/xarray) for numerical analysis. It allows easy access to
 the functionalities of these packages through interfacing or composition, and quick inter-operability through object conversion.
 
+The RichDEM tool, distributed  under the GNU GPL v3.0 license, is used solely for testing purposes and is not required
+for the operation of xDEM. The xDEM tool itself is distributed under the MIT license
+
 If you are looking for an accessible Python package to write the Python equivalent of your [GDAL](https://gdal.org/) command lines, or of your
 [QGIS](https://www.qgis.org/en/site/) analysis pipeline **without a steep learning curve** on Python GIS syntax, xDEM is perfect for you! For more advanced
 users, xDEM also aims at being efficient and scalable by supporting lazy loading and parallel computing (ongoing).

doc/source/terrain.md

@@ -10,9 +10,7 @@ and tested for consistency against [gdaldem](https://gdal.org/programs/gdaldem.h
 ## Quick use
 
 Terrain attribute methods can either be called directly from a {class}`~xdem.DEM` (e.g., {func}`xdem.DEM.slope`) or
-through the {class}`~xdem.terrain` module which allows array input. If computational performance
-is key, xDEM can rely on [RichDEM](https://richdem.readthedocs.io/) by specifying `use_richdem=True` for speed-up
-of its supported attributes (slope, aspect, curvature).
+through the {class}`~xdem.terrain` module which allows array input.
 
 ## Slope