rewrite design.md

[ericbuckley]

Summary

Rewrite the user guide docs/site/design.md document to have the following 3 subsections; Algorithm, Business Rules and MPI.

Acceptance Criteria

• Clear and testable conditions for the completion of the task.
• Another condition that defines task completion.

Details / Tasks

The Algorithm section should talk in detail about link.py and how the 3 pass (blocking, evaluation and clustering) steps work together to determine the results of a linkage request.

The Business rules section should talk about the high level differences in what a user can configure vs what is set by code. This doesn't need to list all the options available to them (we can just link to the reference.md), it should just explain the differences.

The MPI is more or less what is already that document, and should explain the database schema design and how it works with the above two sections.