Finalizes Producer section. A little clean up of bulkimports.

These changes represent my propsed draft of the producer section. I've updated the text and improved the API Usage section with greater details. I've also filled in the library class section.

This change also makes some alterations to bulk-imports to improve the section. It still needs another iteration, but is less dire.

Signed-off-by: Christophe Taton <taton@wibidata.com>
Adds Draft of Music Tutorial.
DOCS-40. Increase width of content in docs.