Improve documentation structure and examples

[anapaulagomes]

The team of Model Bakers is preparing a new repo with examples of usage and also reviewing our current documentation. We noticed that part of the issues we get here are related to the lack of documentation. Some features aren't even in the docs yet. 😅

This issue is a place to write down the places where we think we need to document better. Suggestions are always welcome.

To cover on the docs:

• _bulk_create
• Callable and iterable attribute values Document callable and iterable attribute values #27
• Showcase section (projects or orgs that are using Model Bakery)

Our repo with examples is available here: https://github.com/model-bakers/farm-app