BIM docs: add 1.2 feature notes (Windows, Stairs, DrawingView, IfcExplorer)

[mkcash]

Summary

• Updates 4 BIM Workbench wiki pages with FreeCAD 1.2 features:

1. BIM_Windows – Sliding Door preset (FreeCAD/FreeCAD#27375), task panel options box for editing window properties with expression-aware fields (#27381), and LinkGroups support for window types to avoid out-of-scope subvolume warnings (#27420).

2. Arch_Stairs – Stair railings now follow stair visibility changes, including visibility inherited from parent objects such as levels (#29173).

3. BIM_DrawingView – Cut lines now use the default line width and cut areas line thickness ratio preferences for thicker cut-line output (#30149).

4. BIM_IfcExplorer – Native IFC objects expose the full active-schema class list in the {{Incode|IfcClass}} property, including IFC2X3 type families (#28994).

Validation

• git diff --check HEAD^ HEAD

Addresses #26.

[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.