(chore): generate 0.12.0 release notes (#2031)

Author: ilan-gold

docs/release-notes/0.12.0.md

@@ -0,0 +1,6 @@
+(v0.12.0)=
+### 0.12.0 {small}`2025-07-16`
+
+### Bug fixes
+
+- Fix {func}`anndata.experimental.backed.Dataset2D.reindex` internal setting {user}`ilan-gold` ({pr}`2018`)