📖 Add /agenda redirect in book

For this iteration, we'll keep using the 2023 agenda until the 2024 and
the historical ones are available somewhere safe.

Signed-off-by: Vince Prignano <vincepri@redhat.com>

docs/book/book.toml

@@ -12,6 +12,8 @@ additional-css = ["theme/css/custom.css"]
 
 [output.html.redirect]
 "/tasks/upgrade.html" = "/tasks/upgrading-cluster-api-versions.html"
+"/agenda.html" = "/agenda/2023.html"
+"/agenda/2023.html" = "https://docs.google.com/document/d/1ushaVqAKYnZ2VN_aa3GyKlS4kEd6bSug13xaXOakAQI"
 
 [preprocessor.tabulate]
 command = "./util-tabulate.sh"