Add class documentation for the bulk import models

🌳 What does this MR do?

Following up from a discussion (!42978 (comment 429448364)) where the BulkImport* models were created. We found that the intended structure of these models is not super obvious just from reading this code.

Add some class documentation to these models so that anyone new to the domain can more easily understand the intention behind them.

Eventually, as this feature develops, we can also add contribution guides to our main docs which includes the high level architecture.

🔍 Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• ~~Label as security and @ mention @gitlab-com/gl-security/appsec~~
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team