doc: Adding new architecture document with initial "Overview of modules" section

doc/source/arch_overview.rst

+
+
+Overview of modules
+===================
+Below is a basic overview of the modules, what they are for, and generally
+what their stacking order is internally in BuildStream.
+
+.. image:: images/arch-overview.svg

doc/source/index.rst

@@ -20,6 +20,7 @@ Later sections provide detailed information on BuildStream internals.
    main_using
    main_core
    CONTRIBUTING
+   main_architecture
 
 For any other information, including `how to install BuildStream <https://buildstream.build/install.html>`_,
 refer to `the BuildStream website <https://buildstream.build>`_.

doc/source/main_architecture.rst

+
+
+Architecture
+============
+This section provides details on the overall BuildStream architecture.
+
+
+.. toctree::
+   :maxdepth: 2
+
+   arch_overview