docs: comprehensive external add-on documentation (#331)

[lqkhanh295]

(External Add-on Documentation) by comprehensively organizing and updating the documentation for external add-ons, ensuring it is consistent with the current wiki style.

Compliance with Maintainer Guidelines:

• No T tags added: All new content has been added as plain wikitext without any or tags.
• No translation files touched: Only files in the root wiki/ directory were modified.
• No existing tags removed: Existing T tags and translation markers were kept exactly as they were to prevent any regressions.
• Sources and Reasons: All changes are documented individually below with their sources.

Changes Made:

1. wiki/External_workbenches.wikitext

• Reason: This file previously contained an obsolete warning directing users to an external website. To fulfill the requirement that ''The locations where each individual document was created should be identified and added to the wiki'', this page has been updated to serve as the definitive index for external workbenches on the wiki. The obsolete warning was removed.
• Reason: Added a new section ''Installing external workbenches'' to explain Addon Manager usage and manual installation.
• Reason: Added a new section ''The Addons Index'' to explain curated vs non-curated addons.
• Source: FreeCAD Addons Repository (Index.json documentation) and FreeCAD Developers Handbook.

2. wiki/Macros.wikitext

• Reason: The documentation for macros was scattered. Added a ''Macro repositories'' section to explain the official FreeCAD-macros repository, the Macros recipes page, and the Addon Manager index.
• Reason: Added ''Macro categories'' and ''Installing macros'' sections to properly guide users on how to install and find macros in one centralized location.
• Source: FreeCAD Macros GitHub README and existing scattered wiki pages (e.g., How_to_install_macros).

3. wiki/Addon.wikitext

• Reason: In the ''Different types'' section, the link for ''Workbenches'' previously pointed to an external website (addons.php). It has been updated to point to the newly updated [[External_workbenches]] wiki page, bringing the documentation back into the wiki ecosystem as requested.
• Source: Wiki internal linking guidelines.

[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.