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).

Start in the User Guide's drive documentation and review the attached screenshot for the “Open in” flow. Add a section explaining how uploaded compatible documents can be opened in the CryptPad document, presentation, sheet, and code applications, then verify that the wording matches the interface shown.

Written by the indexing model from the issue text.