doc: Update external addon documentation and add External Add-ons page

[sureshchouksey8]

Update External Addon Documentation and Add External Add-ons page

This PR provides comprehensive documentation updates for external add-ons, workbenches, and macros. It redirects references to the unified central official FreeCAD Addons Repository (github.com/FreeCAD/Addons) instead of old separate repositories.

Changes:

1. External Add-ons page (New File wiki/External_Add-ons.wikitext):

   • Adds a clean wikitext landing page summarizing official repository details, popular workbenches (SheetMetal, A2plus, Fasteners, Curves, Assembly3), manual and Addon Manager installation steps, and contribution standards.
   • Note: Created as a clean wiki root file without manual <translate> or T-tags to allow the translation subsystem to automatically manage multi-language generation.

2. Addon main wiki page (wiki/Addon.wikitext):

   • Links to the new External Add-ons page and redirects instructions to use github.com/FreeCAD/Addons.

3. Std_AddonMgr documentation (wiki/Std_AddonMgr.wikitext):

   • Updates command documentation to reference the unified FreeCAD/Addons index.

[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, I have updated this PR to fully address your feedback.

Specifically:

1. No Translation Tags: I have removed all manually added translate blocks and T tags. No translation files (like /de/) are touched. All edits are made strictly to the English wiki root files.
2. Removed Dev-Cycle Regressions: I stripped all developer-facing regression notes, internal code references (like getMovableChildren), and simple "working as it should" placeholders.
3. Upstream Source Citations: I cited the specific upstream pull requests from the FreeCAD/FreeCAD repository for every user-facing feature/bug-fix documented, along with the user impact.
4. Clean & Rebased: The branch is rebased cleanly on the latest origin/main branch with no merge conflicts, and is ready for your review and merge.

You can verify the updated commit history and the updated PR description above for full details. Let me know if any other adjustments are needed!