docs: explain pixi-build-python backend pin

Co-authored-by: swenger <603549+swenger@users.noreply.github.com>

Author: Copilot

template/pixi.toml.jinja

@@ -9,6 +9,7 @@ preview = ["pixi-build"]
 name = "{{ project_slug }}"
 [package.build.backend]
 name = "pixi-build-python"
+# Keep this pinned so `exclude-newer = "7d"` does not pick a too-recent backend release.
 version = "0.8.3"
 [package.host-dependencies]
 python = ">={{ minimal_python_version.replace('py3', '3.') }}"