Sdg23 add documentation page

pages/documentation.md

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+---
+layout: page
+title: Documentation
+order: 3
+---
+
+MDAnalysis is an open-source, available under the [GNU Lesser General Public License, version 2.1 or later (LGPLv2.1+)](https://www.gnu.org/licenses/lgpl-2.1.html). MDAnalysis provides **comprehensive documentation** to help users at all levels—whether you're just getting started, developing new tools, or contributing to the project. From **step-by-step tutorials** to an in-depth **API reference**, these resources will guide you through installing, using, and extending MDAnalysis.
+
+## User Guide
+The [User Guide]({{ site.docs.userguide.url }}) is the best place for new users to start. It includes:
+
+- [Installation instructions]({{ site.docs.userguide.url }}/stable/installation.html)
+- Tutorials and [examples]({{ site.docs.userguide.url }}/stable/examples/README.html)
+- Detailed explanations of MDAnalysis functionalities (see for example the [`AtomGroup`]({{ site.docs.userguide.url }}/stable/atomgroup.html)) 
+
+## API Reference
+The [API Reference]({{ site.docs.mdanalysis.url }}) offers detailed documentation on all MDAnalysis modules, classes, and functions—perfect for developers and advanced users.
+
+## MDAnalysis Paper
+Learn about the design philosophy and scientific impact of MDAnalysis in the [MDAnalysis paper]({{ site.baseurl }}/pages/citations#Gowers2016).
+
+---
+
+If you need help or have ideas, visit our [Community]({{ site.baseurl }}/pages/community) page to learn about available discussion channels.
+