docs: Move changelog to reference/

why: Changelog is reference material, not onboarding. Grouping it
with API docs and compatibility under reference/ is cleaner.
what:
- Move docs/history.md to docs/reference/changelog.md
- Create docs/reference/index.md with toctree

Author: tony

docs/history.md docs/reference/changelog.mddocs/history.md renamed to docs/reference/changelog.md

@@ -0,0 +1,11 @@
+(reference)=
+
+# Reference
+
+```{toctree}
+:maxdepth: 2
+
+api/index
+compatibility
+changelog
+```