doc: feature list

- put area/line/point features on a single page (pages were very short)
- include a list of all current features

doc/sphinx/user_manual/concepts/feature_list.md

@@ -0,0 +1,41 @@
+(part:user_manual:chap:concepts:sec:feature_list)=
+The current features in GWB
+===========================
+
+As we have learned before, the world in GWB is described by a list of features.
+First looking at a map that illustrates geologic and tectonic features will help you build GWB models. All features are named based on nomenclature typically used in the Earth sciences.
+
+These features can be divided into three different kinds based on how they can be identified on a map: Area features, Line features, and Point features. 
+
+
+
+(part:user_manual:chap:concepts:sec:area_features)=
+# Area features
+
+ In {ref}`part:user_manual:chap:concepts:sec:painting_the_world` we were already introduced to the **area feature**. This is the `oceanic plate` feature we use there. Other examples of area features are the `continental plate` and `mantle layer` features. We will explain how they work in practice in the {ref}`part:user_manual:chap:basic_starter_tutorial:sec:index`. 
+
+The list of all area features is:
+  - "continental plate"
+  - "mantle layer"
+  - "oceanic plate"
+
+
+
+(part:user_manual:chap:concepts:sec:line_features)=
+# Line features
+
+Line features are named after what they look like on a map: a line. These lines still represent 3D objects. For example, in {ref}`part:user_manual:chap:concepts:sec:painting_the_world`, this feature is used in the`subducting plate` feature. We will explain how these features work in practice in the {ref}`part:user_manual:chap:basic_starter_tutorial:sec:index`. 
+
+The list of all line features is:
+  - "fault"
+  - "subducting plate"
+
+
+
+(part:user_manual:chap:concepts:sec:point_features)=
+# Point features
+
+Point features are defined by a single point. Potential future features could be a diapir feature, but that currently does not exist yet.
+
+The list of all point features is:
+  - "plume"

doc/sphinx/user_manual/concepts/index.md

@@ -10,8 +10,6 @@ To understand how to make a World Builder file that creates some interesting ini
 boundless_world
 features
 painting_in_the_world
-area_features
-line_features
-point_features
+feature_list
 constant_angle_in_spherical_domain_issue
 ```