doc/notebooks: add overview README.md (#2416)

* doc/notebooks: add overview README.md

This README.md lists all Jupyter notebooks with their respective titles.
Additionally it is explained how to actually start a Jupyter notebook.

Co-authored-by: Veronica Andreo <veroandreo@gmail.com>

doc/notebooks/README.md

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+## Introduction
+
+### Using the notebooks locally
+
+Clone this repository with `git clone` first. Then, locally start the Jupyter notebook
+server from the command line in the `doc/notebooks/` directory containing the `*.ipynb` files with:
+`jupyter notebook`
+
+This will open a new browser tab or window with a list of the contents of the current
+working directory. Clicking on one of the `*.ipynb` files will start the notebook.
+
+See also the official documentation for [The Jupyter Notebook](https://jupyter-notebook.readthedocs.io/en/latest/).
+
+### Introductory notebooks to GRASS GIS and Jupyter
+
+   - GRASS GIS in Jupyter Notebook with Python and grass.jupyter: [jupyter_example.ipynb](jupyter_example.ipynb)
+   - The grass.jupyter Package: [jupyter_tutorial.ipynb](jupyter_tutorial.ipynb)
+
+## Thematic Jupyter notebooks:
+
+   - Viewshed Analysis: [viewshed_analysis.ipynb](viewshed_analysis.ipynb)
+   - Spatio-Temporal Analysis with grass.jupyter: [temporal.ipynb](temporal.ipynb)
+   - Solar Energy Potential Analysis: [solar_potential.ipynb](solar_potential.ipynb)
+   - GRASS GIS Scripting with Python: [scripting_example.ipynb](scripting_example.ipynb)
+   - Hydrology with GRASS GIS: [hydrology.ipynb](hydrology.ipynb)
+