docs: Add using-files guide and update navigation

- Added `using-files.md` to provide a comprehensive guide on file operations
  in the SDK, covering uploads, downloads, metadata retrieval, and deletions.
- Updated `index.md` to include a link to the new `using-files.md` guide.
- Modified `mkdocs.yml` to reflect the updated navigation structure.

Assisted-by: Codex

Author: datalogics-kam

docs/index.md

@@ -18,6 +18,7 @@ Useful references:
 - [API Lab (interactive testing)](https://pdfrest.com/apilab/)
 - [Getting started guide](getting-started.md)
 - [Client configuration guide](client-configuration.md)
+- [Using files guide](using-files.md)
 
 ## How this Python API relates to pdfRest