Document Mass Properties markers release note

[alexgduarte]

Summary

• Adds the merged Measure: Better ViewProvider for Mass Properties FreeCAD/FreeCAD#29288 Mass Properties marker update to the 1.2 release notes.
• Updates both the source release note page and the English mirror.

Scope

FreeCAD/FreeCAD#29288 is a user-visible measurement improvement: Mass Properties results now use dedicated center of gravity and center of volume markers, and principal axes are labeled 1, 2 and 3.

Related

• Tracks Adding missing information to Release_notes_1.2 page. #30.
• Documents Measure: Better ViewProvider for Mass Properties FreeCAD/FreeCAD#29288.
• Related upstream issues: Mass Properties: Center of gravity and center of volume are hard to see FreeCAD/FreeCAD#28923 and Std_MassProperties puts geometry in the tree by default FreeCAD/FreeCAD#28925.

Duplicate check

• Checked the current release-note files for 29288, center of gravity/volume marker wording, COG, COV, and numbered axes before editing.
• Checked open documentation PRs for FreeCAD/FreeCAD#29288, center of gravity center of volume, and Mass Properties markers; no duplicates were found.

Validation

• git diff --check
• Verified the new release-note entry appears once in each updated release-note file.

[Reqrefusion]

"en" files should not be modified. All translation files should remain untouched.

[Reqrefusion]

This feature was just added, so these other things can be added as well.