docs: move release notes TOC to sub page (#2408)

This allows us to organize the TOC a bit better by adding sections per
major version. We can even increase the maxdepth to 2 now, which looks
great in my opinion.

In addition, the full list of releases is not shown in the sidebar
anymore when viewing a completely different part of the documentation,
which took up more than half of the total sidebar entries.

docs/index.rst

@@ -78,61 +78,7 @@ Several Freifunk communities in Germany use Gluon as the foundation of their Fre
    :caption: Releases
    :maxdepth: 1
 
-   releases/v2021.1.1
-   releases/v2021.1
-   releases/v2020.2.3
-   releases/v2020.2.2
-   releases/v2020.2.1
-   releases/v2020.2
-   releases/v2020.1.4
-   releases/v2020.1.3
-   releases/v2020.1.2
-   releases/v2020.1.1
-   releases/v2020.1
-   releases/v2019.1.3
-   releases/v2019.1.2
-   releases/v2019.1.1
-   releases/v2019.1
-   releases/v2018.2.4
-   releases/v2018.2.3
-   releases/v2018.2.2
-   releases/v2018.2.1
-   releases/v2018.2
-   releases/v2018.1.4
-   releases/v2018.1.3
-   releases/v2018.1.2
-   releases/v2018.1.1
-   releases/v2018.1
-   releases/v2017.1.8
-   releases/v2017.1.7
-   releases/v2017.1.6
-   releases/v2017.1.5
-   releases/v2017.1.4
-   releases/v2017.1.3
-   releases/v2017.1.2
-   releases/v2017.1.1
-   releases/v2017.1
-   releases/v2016.2.7
-   releases/v2016.2.6
-   releases/v2016.2.5
-   releases/v2016.2.4
-   releases/v2016.2.3
-   releases/v2016.2.2
-   releases/v2016.2.1
-   releases/v2016.2
-   releases/v2016.1.6
-   releases/v2016.1.5
-   releases/v2016.1.4
-   releases/v2016.1.3
-   releases/v2016.1.2
-   releases/v2016.1.1
-   releases/v2016.1
-   releases/v2015.1.2
-   releases/v2015.1.1
-   releases/v2015.1
-   releases/v2014.4
-   releases/v2014.3.1
-   releases/v2014.3
+   releases/index
 
 License
 -------

docs/releases/index.rst

+Release Notes
+=============
+
+.. toctree::
+   :caption: Gluon 2021.1
+   :maxdepth: 2
+
+   v2021.1.1
+   v2021.1
+
+.. toctree::
+   :caption: Gluon 2020.2
+   :maxdepth: 2
+
+   v2020.2.3
+   v2020.2.2
+   v2020.2.1
+   v2020.2
+
+.. toctree::
+   :caption: Gluon 2020.1
+   :maxdepth: 2
+
+   v2020.1.4
+   v2020.1.3
+   v2020.1.2
+   v2020.1.1
+   v2020.1
+
+.. toctree::
+   :caption: Gluon 2019.1
+   :maxdepth: 2
+
+   v2019.1.3
+   v2019.1.2
+   v2019.1.1
+   v2019.1
+
+.. toctree::
+   :caption: Gluon 2018.2
+   :maxdepth: 2
+
+   v2018.2.4
+   v2018.2.3
+   v2018.2.2
+   v2018.2.1
+   v2018.2
+
+.. toctree::
+   :caption: Gluon 2018.1
+   :maxdepth: 2
+
+   v2018.1.4
+   v2018.1.3
+   v2018.1.2
+   v2018.1.1
+   v2018.1
+
+.. toctree::
+   :caption: Gluon 2017.1
+   :maxdepth: 2
+
+   v2017.1.8
+   v2017.1.7
+   v2017.1.6
+   v2017.1.5
+   v2017.1.4
+   v2017.1.3
+   v2017.1.2
+   v2017.1.1
+   v2017.1
+
+.. toctree::
+   :caption: Gluon 2016.2
+   :maxdepth: 2
+
+   v2016.2.7
+   v2016.2.6
+   v2016.2.5
+   v2016.2.4
+   v2016.2.3
+   v2016.2.2
+   v2016.2.1
+   v2016.2
+
+.. toctree::
+   :caption: Gluon 2016.1
+   :maxdepth: 2
+
+   v2016.1.6
+   v2016.1.5
+   v2016.1.4
+   v2016.1.3
+   v2016.1.2
+   v2016.1.1
+   v2016.1
+
+.. toctree::
+   :caption: Gluon 2015.1
+   :maxdepth: 2
+
+   v2015.1.2
+   v2015.1.1
+   v2015.1
+
+.. toctree::
+   :caption: Gluon 2014.4
+   :maxdepth: 2
+
+   v2014.4
+
+.. toctree::
+   :caption: Gluon 2014.3
+   :maxdepth: 2
+
+   v2014.3.1
+   v2014.3
+