Add 2.12 changelog to docsite (#16136)
Add 2.12 changelog to docsite (#16136)

[ci skip-rust]
[ci skip-build-wheels]
pantsbuild/pants,benjyw/pants,pantsbuild/pants,benjyw/pants,benjyw/pants,pantsbuild/pants,pantsbuild/pants,benjyw/pants,pantsbuild/pants,pantsbuild/pants,pantsbuild/pants,benjyw/pants,benjyw/pants,benjyw/pants