Add document explaining the use case of building your own DSL

[aaguiarz]

The fact that our model is a JSON enables interesting use cases where OpenFGA adopter can create their own DSLs / UIs to create models. We should document this better.

A few use cases:

• Automatically adding permissions for setting relations (e.g. 'can_set_viewer_relation')

• In a multi-tenant app, enable each B2B customer to redefine permissions with different rewrites, e.g. the model would have:

define can_view : reader or reader from parent

A specific customer could want to change that definition, to:

define can_view : reader or reader from parent or support_engineer from organization

Note that they should not add new permissions, or add new assignable permissions. Just change the rewrites.

Each customer would end up with a different authorization model ID that can be used to evaluate permissions. This would also show the benefits of having multiple authorization models for a specific store.