Require imagine/imagine by default or explain why in the doc

I just installed the bundle on a new project could not get the imagine processing to work.

All I got as an error was:

> The media "pdfs/33b29eb1-6d55-4a0e-ba96-b1793a096879.jpg" from the library "default" cannot be processed for the variation "joli-media-easy-admin-large" (mime type "image/jpeg", format "jpeg", 103429 bytes): no processor worked for this variation

We have no idea what processor have been tried, and the error is difficult to trace back. Would be nice to have some kind of list?

It's all about a missing dependency. The **default processors requires Imagine** (I made no changes to the default config).

Solution is:

```
composer req imagine/imagine
```

So in my opinion this dependency should be:

- installed by default
- or more clearly suggested in the documentation (only the APT INSTALL for binaries is suggested, maybe add the COMPOSER as well?)

Start by checking the bundle's default processor configuration and Composer dependency declarations, then compare them with the documented installation steps. Confirm how the default configuration behaves without imagine/imagine and decide whether the dependency or its Composer installation instructions need to be added. Done means a fresh project can process the default variation or the required dependency is clearly documented.

Written by the indexing model from the issue text.