Improve Upload documentation

[eteubert]

Uploading is currently documented in https://github.com/podlove/radiator/blob/master/guides/Files.md This will be out of date once the Audio restructuring is done.

It would be good to include a JS example on how to upload using gql.

Have a look if this is something to pursue:

• https://github.com/jaydenseric/graphql-upload
• https://github.com/jaydenseric/graphql-multipart-request-spec