docs(known-instances): Add FreeCAD Wiki to known instances (#414)

Update to #407 in accordance with https://blog.freecad.org/2025/04/30/forum-and-wiki-performance-anubis-deployed/

Signed-off-by: Thomas Schuster <twihno@gmail.com>

docs/docs/user/known-instances.md

@@ -31,7 +31,11 @@ This page contains a non-exhaustive list with all websites using Anubis.
 - https://hydra.nixos.org/
 - https://codeberg.org/
 - https://www.cfaarchive.org/
-- https://forum.freecad.org/
+- <details>
+  <summary>FreeCAD</summary>
+  - https://forum.freecad.org/
+  - https://wiki.freecad.org/
+  </details>
 - <details>
   <summary>ScummVM</summary>
   - https://forums.scummvm.org/