Add a section in the User Guide about “Open in” for compatible documents

The drive documentation is missing a section for “Open in” when uploading a file that can be open in a CryptPad application (doc, presentation, sheet & code).

<img width="295" height="500" alt="Image" src="https://github.com/user-attachments/assets/2aff8210-ae98-4962-80f2-a164e346637c" />