Add code outline to architecture docs

Author: jfrost-mo

docs/source/contributing/architecture.rst

@@ -133,3 +133,67 @@ metplus_point_stat
 ~~~~~~~~~~~~~~~~~~
 
 These apps are not currently used, but aim to integrate METplus in the workflow.
+
+Code outline
+------------
+
+The code of CSET lives in the ``src/CSET/`` directory, arranged as follows:
+
+src/CSET
+~~~~~~~~
+
+.. code-block:: text
+
+    src/CSET
+    ├── cset_workflow  # Detailed below for clarity.
+    ├── operators
+    │   ├── __init__.py  # Code for executing ("baking") recipes.
+    │   ├── _colorbar_definition.json  # Default colourbar definitions.
+    │   ├── _plot_page_template.html  # Template for diagnostic output page.
+    │   ├── _stash_to_lfric.py  # Mapping between STASH codes and LFRic variable names.
+    │   ├── _utils.py  # Common utility code for operators.
+    │   └── ...  # Then lots of operators, as described above.
+    ├── recipes
+    │   ├── loaders
+    │   │   ├── __init__.py  # Imports all loaders to make available to the rest of CSET.
+    │   │   └── ...  # Then lots of loaders, as described above.
+    │   ├── __init__.py  # Code for parbaking recipes.
+    │   └── ...  # Then lots of recipes, as described above.
+    ├── __init__.py  # CLI entrypoint. Sets up logging, parses arguments, etc.
+    ├── __main__.py  # Allows running `python -m CSET`.
+    ├── _common.py  # Common utility code.
+    ├── extract_workflow.py  # Implementation of `cset extract-workflow`.
+    └── graph.py  # Implementation of `cset graph`.
+
+src/CSET/cset_workflow
+~~~~~~~~~~~~~~~~~~~~~~
+
+.. code-block:: text
+
+    src/CSET/cset_workflow
+    ├── app  # Contains the rose apps described above.
+    │   ├── assign_model_colours
+    │   │   ├── bin
+    │   │   │   └── assign_model_colours.py  # Executable for app.
+    │   │   └── rose-app.conf  # Rose app configuration. Mostly sets the executable.
+    │   └── ...  # Lots more rose apps in here.
+    ├── bin  # Files in bin are automatically on the workflow's PATH.
+    │   └── app_env_wrapper  # Wrapper script to run things in the conda environment.
+    ├── includes  # Deprecated; Use loaders instead now.
+    ├── lib  # Available for import into cylc's jinja2 templating.
+    │   └── python
+    │       └── jinja_utils.py  # A couple helper functions used in flow.cylc.
+    ├── meta  # Validation and GUI layout for user configuration in rose-suite.conf.
+    │   ├── diagnostics
+    │   │   └── rose-meta.conf  # Diagnostic configuration.
+    │   ├── rose-meta.conf  # Automatically generated file, don't edit.
+    │   └── rose-meta.conf.jinja2  # Workflow configuration.
+    ├── opt  # Pre-made configurations for consistent evaluation.
+    │   └── rose-suite-RAL3LFRIC.conf
+    ├── site  # Site-specific cylc configuration.
+    │   └── localhost.cylc
+    ├── flow.cylc  # The main workflow definition detailing what and how tasks are run.
+    ├── install_restricted_files.sh  # Script for installing site-specific files.
+    ├── README.md
+    ├── rose-suite.conf  # User configuration of workflow and diagnostics.
+    └── rose-suite.conf.example  # Blank user configuration to be copied.