[start] Fix onboarding process

Getting rid of anonymous access had a serious impact on the onboarding experience. If you follow the [tutorial](https://www.aem.live/developer/tutorial), you might get stuck in several places. Use this issue as a container, trying to identify all issues.

Entry point is https://da.live/start or step after the e AEM Code Sync GitHub App is installed.

## Scenario 1 - no config for this org

https://github.com/adobe/da-live/issues/415 handles the config creation.
Then, if you select some demo content:

- [x] Issue: previewing the content fails of https://github.com/adobe/helix-admin/issues/3355. Everything is created on the helix (site config with email address)
- [ ] Issue: helix config uses the github email address of the user which frequently is not the "business" email address. Their might be email address mismatches. TODO: user needs to be informed.

## Scenario 2 - a config exists

### Current user not allowed to write config

- [ ] Issue: initial site config cannot be copied because user cannot write it. Sample site comes with block library definition (see https://da.live/config#/aem-sandbox/block-collection/) which is required to have a fully functional editing experience. TODO: inform the user (but how to suggest some actionable steps, how to identify admin that can create and copy/paste the config... ?)

### Current user not allowed to write content

- [ ] Issue: initial content cannot be copied because user cannot write it. TODO: user needs to be informed he needs to request permissions (but to who, how to identify admins, especially if config cannot be read ?)

In general, we need more info to explain the user what has happened, what is the current state, what is potentially broken...

Start at https://da.live/start and follow the linked tutorial, including the step after the AEM Code Sync GitHub App is installed. Reproduce both scenarios and review the related issues, especially #415 and helix-admin #3355, to identify the current failures. Done means the onboarding flow explains the state, failure, and actionable next steps for configuration and content permission problems.

Written by the indexing model from the issue text.