docs: generate docs daily (#3067)

## What

This change fixes the `generate-docs` workflow, so that they'll run
daily as a cron job. The `schedule` property had been erroneously listed
as a sub-property of the `workflow_dispatch` property.

## Why

The way the docs are set up at the moment, they source a lot of their
content from external repos. As it stands, we don't listen for update
events in all of those external dependencies, so we don't know when they
update. We do still want the docs to be reasonably fresh, though.

So to achieve this, we can run this workflow once a day to make sure
that no docs are more than 24 hours old.

.github/workflows/generate-docs.yaml

@@ -8,8 +8,9 @@ on:
       - website/**
 
   workflow_dispatch:
-    schedule:
-      - cron: '@daily'
+
+  schedule:
+    - cron: '0 0 * * *'
 
 jobs:
   build: