File transfer documentation / tests / bugs update

[FyzHsn]

The file transfers can be improved in several ways:

1. Bugs: S3 file transfer related bug in S3 file transfer strategy upload/download file path bug #1276.
2. Documentation: There's a lot of room for improvement when it comes to providing accurate and detailed information to users on how to use specific file transfer strategies. In the concepts page, we need to discuss and clearly explain why passing files=[] is a very convenient design choice for the users.
   • Fix inaccuracies / grammar in the concepts page (issue Update file transfer how-to #1158).
   • Make file transfer concepts section more abstract and move the technical knowledge to the how-to section (issue Update File Transfer concepts / how-to page #1182).
3. Functional tests: One note on the unit tests for the file transfer - it has higher coverage than average and the tests are well written. Especially, @poojithurao's S3 strategy tests are a very good example of mocking boto3 calls. That being said the file transfers are a very important module and functional tests are essentialto ensure that each of the strategies are working end-to-end.