Note the exception with the blog repo

Co-authored-by: Jan Ainali <jan@publiccode.net>
Co-authored-by: clausmullie <claus@publiccode.net>

activities/documentation/move-merge-delete-pages.md

@@ -43,7 +43,9 @@ redirect_from:
     - [url link 3]
 ```
 
-Note: the redirect url should be the file path without `.md` or `.html` extension.
+The redirect url should typically be the file path without `.md` or `.html` extension.
+The exception is for blog posts in the `blog` repository, where the blog post paths include the category and date, and the redirect _should_ include `.html` at the end.
+Thus, for blog posts the redirect needs to be of the form `/category/yyyy/mm/dd/the-post-slug.html`.
 
 ## Redirecting pages