Add detailed upgrade path/steps in documentation

[sergei-maertens]

How to upgrade and which actions need to be done at which point needs to be properly documented.

1. On 3.5, the migrate_catalogi_api_urls --no-dry-run management command must be executed.
   • Any errors reported must be manually resolved.
   • Document types configured on file components must be verified through the registration backend settings for each form that uses objects/ZGW registration backends.
   • Mismatches in catalogues must be addressed.
2. Then, 4.0 can be deployed which will run the remaining automatic updates/migrations.
   • 4.0 will run an upgrade check before making destructive changes:
     • check that there are no more legacy URLs
     • maybe check if there are file upload configuration consistency issues
3. It's a good idea to re-check the file upload components configuration in the new 4.0 UI (variables table) and check that the correct document type is set up