Documentation missing Multipart upload

### Is your feature request related to a problem? Please describe.
Docs is missing description for Multipart uploads.
https://github.com/box/box-windows-sdk-v2/blob/master/docs/files.md#upload-a-file

Boxmultipartupload method found [here](https://github.com/box/box-windows-sdk-v2/blob/895cc91ebc90bd305d676e2687a8ec913aca01fb/Box.V2/Wrappers/BoxMultiPartRequest.cs#L9)

### Describe the solution you'd like
Add BoxMultipart example in docs

### Describe alternatives you've considered
na

### Additional context
na

Start in docs/files.md at the existing “Upload a file” section, then read Box.V2/Wrappers/BoxMultiPartRequest.cs to understand the multipart method referenced by the issue. Add a BoxMultipart usage example to the documentation, with the upload section showing how the method is used.

Written by the indexing model from the issue text.