docs: update docs to mention that files are extracted from source where source code is available

Recent Dokku changes invalidate a bunch of docs around where files need to be placed in order for Dokku to respect them. This doc change clarifies where files are extracted from in cases where source code is available, which should hopefully make users less confused about how the system works.

See https://railsnotes.xyz/blog/deploying-ruby-on-rails-with-dokku-redis-sidekiq-arm-docker-hetzner for the inspiration - I was reading through it and was like 'these docs are definitely incorrect...'.

docs/processes/scheduled-cron-tasks.md

@@ -35,6 +35,8 @@ A cron entry takes the following properties:
 
 Zero or more cron commands can be specified per app. Cron entries are validated after the build artifact is created but before the app is deployed, and the cron schedule is updated during the post-deploy phase.
 
+See the [app.json location documentation](/docs/advanced-usage/deployment-tasks.md#changing-the-appjson-location) for more information on where to place your `app.json` file.
+
 #### Task Environment
 
 When running scheduled cron tasks, there are a few items to be aware of: