Migration guide: Update automatic unique_id assignment examples

[EwoutH]

Improve the automatic unique_id assignment section in the migration guide with clearer before/after examples. Show both positional and keyword argument cases for agent initialization and super() calls. Add important notes about deprecation warnings and custom ID storage.

It now reads:

Automatic assignment of unique_id to Agents

In Mesa 3.0, unique_id for agents is now automatically assigned, simplifying agent creation and ensuring unique IDs across all agents in a model.

1. Remove unique_id from agent initialization:

   # Old
   agent = MyAgent(unique_id=unique_id, model=self, ...)
   agent = MyAgent(unique_id, self, ...)
   agent = MyAgent(self.next_id(), self, ...)
   
   # New
   agent = MyAgent(model=self, ...)
   agent = MyAgent(self, ...)

2. Remove unique_id from Agent super() call:

   # Old
   class MyAgent(Agent):
       def __init__(self, unique_id, model, ...):
           super().__init__(unique_id, model)
   
   # New
   class MyAgent(Agent):
       def __init__(self, model, ...):
           super().__init__(model)

3. Important notes:

   • unique_id is now automatically assigned relative to a Model instance and starts from 1
   • Model.next_id() is removed
   • If you previously used custom unique_id values, store that information in a separate attribute

• Ref: PR #2226, PR #2260, Mesa-examples PR #194, Issue #2213

[tpike3]

LGTM