Resolve "User testing follow up (Documentation)"

Closes #202 (closed)

Adds required/optional to fields in the AWS Product Sender docs, also adds docs for the ProductsCreatedAfterClient.

There was a broken link with the "sending" docs seen here if you click on "back to user guide". I split the sender.md file into multiple files like the other sections and I think this fixes it.

You can see the site here:

https://ghsc.code-pages.usgs.gov/users/bhawthorne/pdl/userguide/

@emartinez @ehunter @khaynie @hhunsinger @jdbrown @zcloutet @rking