doc: document missing BIM workbench features (fixes #26)

[sureshchouksey8]

This PR implements the documentation updates for issue #26 (BIM Workbench documentation updates).

All updates are restricted strictly to English wiki root files, with no translation files/directories modified, and each new user-facing feature or change has been cited with its upstream FreeCAD pull request source.

PayPal: https://paypal.me/sureshc26

[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. "It's working as it should" isn't enough to be included. The reasons for each change made, along with their sources, should be explained individually.

[sureshchouksey8]

Hi @Reqrefusion,

Thank you for your feedback! Here is the breakdown of the changes made in this Pull Request, including their rationale and official sources:

Summary of Changes & Rationale

File	Change / Addition	Rationale	Source
wiki/Release_notes_26.3.wikitext	Added note about removing NorthDeviation property reference in BIM Project Manager.	Documents removal of obsolete and unused properties to keep the project manager metadata clean.	FreeCAD PR #28913
wiki/Release_notes_26.3.wikitext	Added note about fixing Arch CutPlane deselection issue in BIM.	Documents the fix for a UI bug where CutPlane objects could not be deselected.	FreeCAD PR #29699
wiki/Release_notes_26.3.wikitext	Added note about fixing unsupported base objects handling for ArchWalls.	Documents the bug fix ensuring robust validation of base objects for ArchWalls.	FreeCAD PR #29810

Note on Translation Files & Tags

• No translation files (such as files under subfolders like /pl/ or /fr/) were modified in this PR.
• Only the root English documentation file wiki/Release_notes_26.3.wikitext was modified.
• No new translation tags (<!--T:...--> or <translate>) were manually added.

For tracking/bounty purposes, preferred payment: https://paypal.me/sureshc26

[Reqrefusion]

Does the user need to know this?