docs: add change log

Author: boblat

changelog.d/20260122_170552_bobby.lat_arc4_contract.md

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+<!--
+A new scriv changelog fragment.
+
+Uncomment the section that is right (remove the HTML comment wrapper).
+For top level release notes, leave all the headers commented out.
+-->
+
+<!--
+### Removed
+
+- A bullet item for the Removed category.
+
+-->
+
+### Added
+
+- Added `algopy.Contract` as an alias of `algopy.arc4.ARC4Contract` for convenience.
+
+### Changed
+
+- Non-ARC-4 contract class is renamed as `algopy.BaseContract`
+
+<!--
+### Deprecated
+
+- A bullet item for the Deprecated category.
+
+-->
+<!--
+### Fixed
+
+- A bullet item for the Fixed category.
+
+-->
+<!--
+### Security
+
+- A bullet item for the Security category.
+
+-->