EZP-23096: As a developer, I want to have a clear spec/examples of the PlatformUIBundle semantic configuration and how external bundles can modify it

JIRA: https://jira.ez.no/browse/EZP-23096

# Description

This PR adds a design document on how the PlatformUIBundle semantic configuration should be written and how external bundles can inject custom modules in the Platform UI App code.

Any input is of course welcome, especially on the 2 open questions at the end of the document.
Unless you expect that *all* configuration for PlatformUI must be siteaccess aware (now and in the future), you're missing a key here.
Should be more somthing like:

```yaml
platformui:
    system:
        my_siteaccess:
            yui:
                # ...
```