docs: README and ARCHITECTURE diagram

Author: bradenmacdonald

openedx_catalog/ARCHITECTURE.md

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+# Catalog App Architecture Diagram
+
+Here's a visual overview of how this app relates to other apps.
+
+(_Note: to see the diagram below, view this on GitHub or view in VS Code with [a Markdown-Mermaid extension](https://marketplace.visualstudio.com/items?itemName=bierner.markdown-mermaid) enabled._)
+
+```mermaid
+---
+config:
+  theme: 'forest'
+---
+flowchart TB
+    Catalog["**openedx_catalog** (CourseRun, CatalogCourse plus core metadata models, e.g. CourseSchedule. Other metadata models live in other apps but are 1:1 with CourseRun.)"]
+    Content["**openedx_content**<br>The content of the course. (publishing, containers, components, media)"]
+    Organizations["**edx-organizations** (Organization)"]
+    Enrollments["**platform: enrollments** (CourseEnrollment, CourseEnrollmentAllowed)"]
+    Modes["**platform: course_modes** (CourseMode)"]
+    Catalog <-. "Direction of this relationship TBD." .-> Content
+    Catalog -- References --> Organizations
+    Enrollments -- References --> Modes
+    Enrollments -- References --> Catalog
+
+    style Enrollments fill:#ccc
+    style Modes fill:#ccc
+    style Organizations fill:#ccc
+
+    Pathways["<a href='https://openedx.atlassian.net/wiki/spaces/OEPM/pages/5148147732/Brief+Modular+Content+Delivery+-+Platform+Strategy'>**openedx_pathways**</a> (Pathway, PathwaySchedule, PathwayEnrollment, PathwayCertificate, etc.)"]
+    Pathways -- References --> Catalog
+
+    style Pathways fill:#c0ffee,stroke-dasharray: 5 5
+
+    FutureCatalog["Future discovery service - learner-oriented, pluggable, browse/search courses and programs"] -- References --> Catalog
+    FutureCatalog <-- Plugin API --> Pathways
+    style FutureCatalog fill:#ffc0ee,stroke-dasharray: 5 5
+```

openedx_catalog/README.rst

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+Learning Core: Catalog App
+==========================
+
+Overview
+--------
+
+The ``openedx_catalog`` Django apps provides core models to represent all courses in the Open edX platform. Higher-level apps can build on these models to implement features like enrollment, grading, scheduling, exams, and much more.
+
+Motivation
+----------
+
+The existing ``CourseOverview`` model in ``openedx-platform`` is derived from various places, but mostly from the metadata fields of the root ``Course`` object stored in modulestore (MongoDB) for each course. As we slowly transition toward storing course content fully in Learning Core (in ``openedx_content``), we want to move to storing all course data and metadata in these sort of MySQL models. We're creating this new ``CourseRun`` model in ``openedx_catalog`` to support these goals:
+
+1. Provide a core model to represent each course, for foreign key purposes.
+2. To allow provisioning placeholder courses before any content even exists.
+3. To be much simpler and more performant than ``CourseOverview`` was (far fewer fields generally, fewer legacy fields, integer primary key).
+4. Perhaps to provide a transition mechanism, a pointer than can point either to modulestore content or learning core content, as we transition content storage.
+
+Architecture
+------------
+
+See `the architecture diagram <./ARCHITECTURE.md>`__. (Because we use RST for all Python READMEs, it cannot be embedded here directly.)