Add more info on Duplicate behavior doc

Doc to improve:

• https://doc.nuxeo.com/client-apps/nuxeo-drive-direct-transfer/#duplicates-behavior

Add a technical section about how filemanager works.
Then two use cases:

• hash file already exist
• filename already exist

For the first case, we may want no to create it again, for the second case, it might be a case of wanting to upgrade the version, etc...