ADR files hierarchy

Hello guys,

There was a remark on this ADR 
https://github.com/apache/james-project/pull/2894 expressed by the 
community regarding proposing restructuring the way we organize ADRs.

It might be better to not mix the ADRs belonging to James Core with the 
ADRs related to some James extensions (like the Deleted Message Vault).

We seem to overall agree that it would be easier to allow new developers 
for example to catch up with James core architecture as an all without 
over-complexify it for them with the extensions.

However we seem to be less sure on how to manage that.

Spark of initial idscussion in the folloowing PR comment: 
https://github.com/apache/james-project/pull/2894#discussion_r2658835617

I think the easiest way IMO would be to divide the `src/adr` folder into 
subfolders?  Something like:

```
src/
   - adr/
     + core/
     + extensions/
[...]
```

Then it would be easy to maintain ADRs for both core and extensions, 
easy for everybody to find, easy to add new ones... Not confusing as 
well on what is core and what is extension ADR...

Well that's my proposition anyways, opened to discussion !

Best regards,

Rene.