docs(known-instances): add extensions.typo3.org (#691)

Signed-off-by: Lothar Serra Mari <mail@serra.me>

docs/docs/user/known-instances.md

@@ -44,6 +44,7 @@ This page contains a non-exhaustive list with all websites using Anubis.
 - https://squirreljme.cc/
 - https://gitlab.postmarketos.org/
 - https://wiki.koha-community.org/
+- https://extensions.typo3.org/
 - <details>
   <summary>FreeCAD</summary>
   - https://forum.freecad.org/