Trac ticket number

ticket-18296

Branch description

Given the following project structure, where apps is a directory:

.
├── apps
├── manage.py
└── myproject
    ├── __init__.py
    ├── asgi.py
    ├── settings.py
    ├── urls.py
    └── wsgi.py

If I run:

$ django-admin startapp app1 apps/app1
CommandError: Destination directory '/Users/bruno/.../myproj/apps/app1' does not exist, please create it first.

This can cause some friction with beginners, and it feels like it would be something that Django could create out of the box.

The code is shared with startproject, so this PR is doing it for both, as I couldn't find a reason why not.

Checklist

• This PR targets the main branch.
• The commit message is written in past tense, mentions the ticket number, and ends with a period.
• I have checked the "Has patch" ticket flag in the Trac system.
• I have added or updated relevant tests.
• I have added or updated relevant docs, including release notes if applicable.
• I have attached screenshots in both light and dark modes for any UI changes.