Expand Project Info documentation. Reorganize General Usage (#4875)

src/blenderbim/docs/users/user_interface.rst

@@ -0,0 +1,16 @@
+User interface
+==============
+
+This section covers the general user interface of BlenderBIM. User interface documentation does not cover any specific use case but provides a general reference of BlenderBIM UI.
+
+The documentation doesn't cover Blender UI, but makes references to `Blender documentation <https://docs.blender.org/manual/en/latest/interface/index.html>`__ where appropriate.
+
+.. toctree::
+   :hidden:
+   :maxdepth: 1
+   :caption: Contents:
+
+   user_interface/topbar
+   user_interface/workspace
+   user_interface/property-editor
+