[DOC] update the documentation to new config and env

part of #306

doc/reference/environment.md

@@ -0,0 +1,18 @@
+# 🦉 Environment 🦉
+
+An environment is an object which contains a [`QWeb` instance](qweb.md). Whenever a root component is created, it is assigned an environment. This environment
+is then automatically given to each sub component (and accessible in the `this.env` property).
+
+```
+    Root
+    /  \
+   A    B
+```
+
+This way, all components share the same `QWeb` instance.
+
+Note: some additional information can be found here:
+
+- [What should go into an environment?](../learning/environment.md)
+- [Customizing an environment](config.md#env)
+