doc: document missing PartDesign workbench features (fixes #79)

[sureshchouksey8]

This PR implements the documentation updates for issue #79 (PartDesign 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.

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[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 Chamfer selection activation.	Documents that the Chamfer tool now automatically activates selection mode upon GUI initialization.	FreeCAD PR #29313
wiki/Release_notes_26.3.wikitext	Added note about expression migration in TwoLengths pads.	Documents improved backwards-compatibility migration for Length parameters in TwoLengths pads.	FreeCAD PR #29247
wiki/Release_notes_26.3.wikitext	Added note about pocket/pad UpToShape calculations.	Documents improved calculation robustness against complex geometric bodies.	FreeCAD PR #29686

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.

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