Backport: docs: add release date for v0.28.1

Signed-off-by: F.N. Claessen <felix@seita.nl>

(cherry picked from commit 7de5279)
Signed-off-by: F.N. Claessen <felix@seita.nl>

Author: Flix6x

documentation/changelog.rst

@@ -4,14 +4,15 @@ FlexMeasures Changelog
 **********************
 
 
-v0.28.1 | September XX, 2025
+v0.28.1 | September 18, 2025
 ============================
 
 Bugfixes
 -----------
 * Fix schema validation in ``PATCH /assets/(id)`` [see `PR #1711 <https://www.github.com/FlexMeasures/flexmeasures/pull/1711>`_]
 * Fixed example values for peak pricing and improved error message for incompatible price units set in the ``flex-context`` [see `PR #1710 <https://github.com/FlexMeasures/flexmeasures/pull/1710>`_]
 
+
 v0.28.0 | September 10, 2025
 ============================