fix: Use correct URL for translation process

Fixes qgis#9234

conf.py

@@ -239,7 +239,7 @@ def calculate_tx_slug(filepath):
     .. only:: html and i18n
 
       .. important::
-        Translation is a community effort `you can join <https://qgis.org/en/site/getinvolved/translate.html#becoming-a-translator>`_.
+        Translation is a community effort `you can join <https://docs.qgis.org/latest/en/docs/documentation_guidelines/do_translations.htm>`_.
         This page is currently translated at |translation progress|.
     """