Add mixed-precision to the changelog

Add mixed-precision to the new features available with tag 4.2.1 (https://sites.nemo-ocean.io/draft-guide/changes.html)

created branch 12-add-mixed-precision-to-the-changelog to address this issue

mentioned in merge request !7 (merged)

mentioned in commit a6c2afde

closed with merge request !7 (merged)

marked this issue as related to #9 (closed)