docs: Use with to open files in multipart examples

[bcyran]

Summary

While reading the docs for multipart file uploads, I wrongly assumed httpx will take care of closing the uploaded files because it was not done manually in the examples. Of course, I understand that the docs never stated this and closing files is usually a caller's responsibility, but I think it's still a good idea to make the examples more practical and less prone to confusion.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.