Write initial Mesa Migration guide (#2257)

Contains a rough outline for a Mesa Migration guide, including Mesa 3.0.

Includes something on Reserved and private variables, Removal of mesa.flat namespace, Automatic increase of the `steps` counter

Will be expanded further - help appreciated!

docs/index.md

@@ -81,6 +81,7 @@ ABM features users have shared that you may want to use in your model
 Mesa Overview <overview>
 tutorials/intro_tutorial
 tutorials/visualization_tutorial
+Migration guide <migration_guide>
 Best Practices <best-practices>
 How-to Guide <howto>
 API Documentation <apis/api_main>

docs/migration_guide.md

@@ -0,0 +1,67 @@
+# Mesa Migration guide
+This guide contains breaking changes between major Mesa versions and how to resolve them.
+
+Non-breaking changes aren't included, for those see our [Release history](https://github.com/projectmesa/mesa/releases).
+
+## Mesa 3.0
+<!-- TODO small introduction-->
+
+_This guide is a work in progress. The development of it is tracked in [Issue #2233](https://github.com/projectmesa/mesa/issues/2233)._
+
+### Reserved and private variables
+<!-- TODO: Update this section based on https://github.com/projectmesa/mesa/discussions/2230 -->
+
+#### Reserved variables
+Currently, we have reserved the following variables:
+  - Model: `agents`, `current_id`, `random`, `running`, `steps`, `time`.
+  - Agent: `unique_id`, `model`.
+
+You can use (read) any reserved variable, but Mesa may update them automatically and rely on them, so modify/update at your own risk.
+#### Private variables
+Any variables starting with an underscore (`_`) are considered private and for Mesa's internal use. We might use any of those. Modifying or overwriting any private variable is at your own risk.
+
+- Ref: [Discussion #2230](https://github.com/projectmesa/mesa/discussions/2230), [PR #2225](https://github.com/projectmesa/mesa/pull/2225)
+
+
+### Removal of `mesa.flat` namespace
+The `mesa.flat` namespace is removed. Use the full namespace for your imports.
+
+- Ref: [PR #2091](https://github.com/projectmesa/mesa/pull/2091)
+
+
+### Automatic assignment of `unique_id` to Agents
+<!-- TODO -->
+
+- Ref: [PR #2226](https://github.com/projectmesa/mesa/pull/2226)
+
+
+### AgentSet and `Model.agents`
+#### AgentSet
+<!-- TODO  -->
+
+#### `Model.agents`
+<!-- TODO  -->
+
+
+### Time and schedulers
+<!-- TODO general explanation-->
+
+#### Automatic increase of the `steps` counter
+The `steps` counter is now automatically increased. With each call to `Model.steps()` it's increased by 1, at the beginning of the step.
+
+You can access it by `Model.steps`, and it's internally in the datacollector, batchrunner and the visualisation.
+
+- Ref: [PR #2223](https://github.com/projectmesa/mesa/pull/2223), Mesa-examples [PR #161](https://github.com/projectmesa/mesa-examples/pull/161)
+
+#### Removal of `Model._time` and rename `._steps`
+- `Model._time` is removed. You can define your own time variable if needed.
+- `Model._steps` steps is renamed to `Model.steps`.
+
+#### Removal of `Model._advance_time()`
+- The `Model._advance_time()` method is removed. This now happens automatically.
+
+<!-- TODO deprecate all schedulers? -->
+
+
+### Visualisation
+<!-- TODO -->