Document BIM classification download source updates

[chinaxiao123]

Summary

• update BIM_Classification so the classification download guidance points readers to the current sources for the newest files
• clarify in Macro_Download_Classifications that the convenience macro can lag behind newer published classification data
• keep the change limited to root wiki/ pages, without touching translation files

Validation

• updated only root wiki pages
• kept existing <translate> structure intact
• based the wording on the newer BIM classification source change from FreeCAD/FreeCAD#30247

Refs #26

[Reqrefusion]

Which FreeCAD PR did this?

[chinaxiao123]

@Reqrefusion The documentation update is based on the BIM classification source changes in FreeCAD/FreeCAD#30247. Specifically:

• The wiki page was updated to reflect the newer classification download source paths introduced in that FreeCAD PR
• The page was clarified to note that the macro may lag behind the newest published classification data

Let me know if you need any further details on the specific changes!

[Reqrefusion]

Should the user know this?