docs: add ADR for pathway catalog and content split

docs/openedx_learning/decisions/0004-pathway-catalog-content-split.rst

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+.. _openedx-learning-adr-0004:
+
+4. Pathways: Split Between Catalog and Content
+==============================================
+
+Status
+------
+
+Draft
+
+Context
+-------
+
+Courses in ``openedx-core`` already separate the *catalog* side (``openedx_catalog``: ``CatalogCourse``,
+``CourseRun`` - what learners browse and enroll against) from the *content* side (``openedx_content`` - what is
+authored, versioned, and published). Pathways have the same two aspects, and the same reasons to keep them apart:
+
+- **Different change rates.** The display name, description shown in the catalog, and SEO metadata are revised
+  frequently and casually. The definition of what a learner must do to complete the Pathway changes rarely and
+  deliberately.
+- **Different people doing the editing.** Catalog data is typically maintained by marketing or communications staff;
+  the Pathway definition is maintained by content authors. Both are visible to learners, so the distinction is about
+  who edits what, not about who can see it.
+- **Different permissions follow from that.** We expect instances to want to let marketing staff update catalog
+  copy without granting them the ability to change what learners must complete, and vice versa. Keeping the two
+  apart makes that possible without inventing field-level permissions.
+- **Auditability.** Progress and credentials must be judged against the definition that was in effect at the time,
+  which requires versioning the definition - but versioning catalog copy would be pure overhead.
+
+Decisions
+---------
+
+1. A Pathway is split into two parts:
+
+   - **Catalog Pathway** - the learner-browsable, enrollable thing. It includes the display name, the description
+     shown in the catalog, SEO metadata, and a **Category**: a student-facing label for the kind of Pathway it is
+     (e.g. "Master's Degree", "Annual Training"). If it is specified, learners see the Category instead of the word
+     "Pathway". The Catalog Pathway is **not versioned**.
+
+   - **Pathway content** - the definition of the Pathway: its Items and its completion criteria. The content is
+     **versioned**, so that we can always tell what the definition was at the moment a learner enrolled or earned a
+     credential.
+
+2. In authoring contexts (Studio, Django admin, code, docs), the terminology is always "Pathway", with the Category
+   shown explicitly. Relabelling is a learner-facing concern of the catalog side only.
+
+3. Learners enroll against the Catalog Pathway. Progress and credential evaluation run against a version of the
+   Pathway content.
+
+Example content of each model:
+
+============================  ==========================
+Catalog Pathway               Pathway content
+============================  ==========================
+Display name                  Pathway Items
+Category                      Completion criteria
+Description
+SEO metadata
+Enrollment
+============================  ==========================
+
+.. Run `dot -Tsvg images/pathway-catalog-content.dot > images/pathway-catalog-content.svg` to regenerate the diagram
+   after making changes to `images/pathway-catalog-content.dot`.
+
+.. image:: images/pathway-catalog-content.svg
+   :alt: Catalog Pathway vs versioned Pathway content
+   :width: 100%
+
+Consequences
+------------
+
+- Catalog edits never create new content versions; definition edits (Items, criteria) always do.
+- Credential and progress records can reference the exact content version in effect at the time, keeping them
+  auditable after the Pathway changes.
+- The unversioned Catalog Pathway can be long-lived even if its content definition is changed significantly over time.

docs/openedx_learning/decisions/images/pathway-catalog-content.dot

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+digraph pathway_catalog_content {
+    rankdir=LR;
+    fontname="Helvetica";
+    node [shape=box, style=rounded, fontname="Helvetica", fontsize=11];
+    edge [fontname="Helvetica", fontsize=10];
+
+    learner [label="Learner", shape=ellipse];
+    catalog [label="Catalog Pathway\n(not versioned)\nname, Category,\nmarketing, SEO"];
+
+    subgraph cluster_content {
+        label="Pathway content (versioned)";
+        fontsize=11;
+        style=dashed;
+        v1 [label="v1: Items, criteria"];
+        v2 [label="v2: Items, criteria"];
+        v1 -> v2 [style=dotted, label="revision"];
+    }
+
+    learner -> catalog [label="browses / enrolls"];
+    catalog -> v2 [label="current definition"];
+}