add change graph

lix/packages/sdk/docs/20-concepts/30-change-graph.md

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+# Change Graph 
+
+```mermaid
+graph TB
+    G["Change G"] --> F["Change D"]
+    F --> B["Change B"]
+    F --> C["Change C"]
+    B --> A["Change A"]
+    C --> A
+```
+
+## Purpose
+
+Represent the relationships and history of changes. 
+
+## Examples
+
+### Dependencies
+
+Change B depends on Change A.
+
+```mermaid
+graph TB
+    G["Change G"]:::unhighlight --> F["Change D"]
+    F["Change D"]:::unhighlight
+    F --> B["Change B"]
+    F --> C["Change C"]:::unhighlight
+    B --> A["Change A"]
+    C --> A
+  
+classDef unhighlight, opacity:0.5;
+```
+
+### Merges
+
+Change C is the result of Change A and Change B.
+
+```mermaid
+graph TB
+    G["Change G"]:::unhighlight --> F["Change D"]
+    F["Change D"]
+    F --> B["Change B"]
+    F --> C["Change C"]
+    B --> A["Change A"]
+    C --> A:::unhighlight
+  
+classDef unhighlight, opacity:0.5;
+```
+
+### History
+
+Change G is the result of Changes D, B, C, and A.
+
+```mermaid
+graph TB
+    G["Change G"] --> F["Change D"]
+    F["Change D"]
+    F --> B["Change B"]
+    F --> C["Change C"]
+    B --> A["Change A"]
+    C --> A
+```
+
+## FAQ 
+
+### Why not call it change history?
+
+The term "history" implies a linear sequence of changes which is incorrect. A lix change graph visualizes the relationships between changes, which includes the history but also includes other relationships such as dependencies and merges.