Add documentation on themes, move some themes around

Docs/Themes/trees.md

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+
+
+ Trees ( trees) 
+----------------
+
+
+
+Map all the trees!
+
+This theme contains the following layers:
+
+
+
+  - [tree_node](../Layers/tree_node.md)
+  - [gps_location](../Layers/gps_location.md)
+  - [gps_location_history](../Layers/gps_location_history.md)
+  - [home_location](../Layers/home_location.md)
+  - [gps_track](../Layers/gps_track.md)
+  - [note_import_tree_node](../Layers/note_import_tree_node.md)
+
+
+Available languages:
+
+
+
+  - en
+  - nl
+  - fr
+  - it
+  - ru
+  - ja
+  - zh_Hant
+  - pl
+  - de
+  - nb_NO
+  - id
+  - hu
+  - ca
+  - es
+  - da
+  - pa_PK
+ 
+
+This document is autogenerated from [assets/themes/trees/trees.json](https://github.com/pietervdvn/MapComplete/blob/develop/assets/themes/trees/trees.json)