Document Assembly simulation playback buttons

[newmattock]

Summary:

• Add a FreeCAD 1.2 release-note entry for the Assembly simulation playback button UI update.

Related bounty or issue:

• Adding missing information to Release_notes_1.2 page. #30
• Assembly simulation playback: make buttons bigger and more distinguishable FreeCAD/FreeCAD#30257

What changed:

• Added one root wiki/Release_notes_1.2.wikitext bullet describing the larger, more distinguishable simulation playback play/step buttons.

Validation:

• Checked current root release notes for existing #30257/playback-control coverage.
• Ran git diff --check -- wiki/Release_notes_1.2.wikitext.

Notes:

• This PR only edits the wiki root release-notes file; it does not modify translation files or add manual translation tags.

[Reqrefusion]

Do not add T tags yourself; only modify the files in the wiki root directory, do not touch the translation files. Please check the latest version and ensure that the information you are adding has not been added previously. Also, be sure to cite the source of your change. Regression corrections should not be added to the documentation. And it should never contain information that the end user shouldn't know.

[newmattock]

Withdrawing this after re-checking the new FreeCAD rules. I missed existing open coverage for the same source change (#134/#314/#316), so keeping another duplicate PR open would add noise. Sorry for the duplicate.