Move doc/global/index to a new toctree section

Author: khaeru

doc/index.rst

@@ -1,8 +1,12 @@
 Tools for MESSAGEix-GLOBIOM models
 **********************************
 
-:mod:`message_ix_models` provides tools for research using the **MESSAGEix-GLOBIOM family of models** developed by the IIASA Energy, Climate, and Environment (ECE) Program and its collaborators.
-This ‘family’ includes single-country and other models derived from the main, global model; all built in the `MESSAGEix framework <https://docs.messageix.org>`_ and on the `ix modeling platform (ixmp) <https://docs.messageix.org/ixmp/>`_.
+:mod:`message_ix_models` provides tools for research using the **MESSAGEix-GLOBIOM family of models**
+developed by the IIASA Energy, Climate, and Environment (ECE) Program and its collaborators.
+This ‘family’ includes the `‘main’ or ‘base’ global model <#index-conceptual>`_
+and derived models including single-country models and `variants with greater sectoral detail <#index-variant>`_.
+All are built using the `MESSAGEix framework <https://docs.messageix.org>`_
+and underlying `ix modeling platform (ixmp) <https://docs.messageix.org/ixmp/>`_.
 
 Among other tasks, the tools allow modelers to:
 
@@ -27,6 +31,29 @@ Among other tasks, the tools allow modelers to:
    whatsnew
    bibliography
 
+.. _index-conceptual:
+
+Conceptual documentation
+========================
+
+This section contains **conceptual and methodological** documentation
+of the ‘base’ or ‘main’ MESSAGEix-GLOBIOM global model,
+describing in detail how it represents global energy systems:
+
+.. toctree::
+   :maxdepth: 2
+   :caption: Conceptual documentation
+   :hidden:
+
+   global/index
+
+- :doc:`global/index`
+
+This is distinct from both
+the *technical* documentation in other sections and pages,
+which describe the code and data used to implement this representation,
+and the modules for various `model ‘variants’ <#index-variants>`_ that have similar, yet distinct, structure.
+
 API reference
 =============
 
@@ -81,12 +108,12 @@ Commonly used classes may be imported directly from :mod:`message_ix_models`.
    api/testing
    api/tests
 
+.. _index-variants:
 
 .. toctree::
    :maxdepth: 2
    :caption: Model variants
 
-   global/index
    buildings/index
    material/index
    transport/index