docs: reorganize toctree sections and remove listeners from concepts

Restructure the docs sidebar into clearer sections:
- Core Concepts: concepts, states, transitions, events, actions, guards
- Runtime: statechart, processing_model, error_handling, async, listeners
- Configuration: behaviour, validations
- Advanced: invoke, models, integrations, weighted_transitions
- Reference: api, diagram, examples, contributing, authors

Remove the Listeners subsection from concepts.md since it now lives
under Runtime, and add a brief note pointing to its dedicated page.

Author: fgmacedo

docs/concepts.md

@@ -37,6 +37,8 @@ Even in this minimal example, the core concepts appear:
 | {ref}`Condition <conditions>` | A guard that allows/blocks a transition | `cond`, `unless`, `validators` parameters |
 | {ref}`Listener <listeners>` | An external observer of the lifecycle | `listeners = [...]` class attribute |
 
+Each concept below introduces the idea briefly; follow the "See also" links
+for the full reference. Listeners are covered in {ref}`their own page <listeners>`.
 
 (concepts-statechart)=
 
@@ -148,19 +150,3 @@ different guard — the first one that passes wins.
 See [](guards.md) for the full reference: `cond`, `unless`, `validators`,
 boolean expressions, and checking enabled events.
 ```
-
-
-(concepts-listeners)=
-
-## Listeners
-
-A {ref}`listener <listeners>` is an external object that observes a state
-machine's lifecycle without modifying its class. Listeners receive the same
-callbacks as the state machine itself — `on_enter_state()`,
-`after_transition()`, and so on — enabling cross-cutting concerns like
-logging, persistence, or telemetry to be attached and removed independently.
-
-```{seealso}
-See [](listeners.md) for the full reference: defining listeners, class-level
-declarations, runtime attachment, the `setup()` protocol, and inheritance.
-```

docs/index.md

@@ -18,25 +18,32 @@ tutorial
 :hidden:
 
 concepts
-statechart
 states
 transitions
 events
 actions
 guards
-listeners
 ```
 
 ```{toctree}
-:caption: Engine
+:caption: Runtime
 :maxdepth: 2
 :hidden:
 
+statechart
 processing_model
 error_handling
 async
-validations
+listeners
+```
+
+```{toctree}
+:caption: Configuration
+:maxdepth: 2
+:hidden:
+
 behaviour
+validations
 ```
 
 ```{toctree}
@@ -47,7 +54,6 @@ behaviour
 invoke
 models
 integrations
-diagram
 weighted_transitions
 ```
 
@@ -57,6 +63,7 @@ weighted_transitions
 :hidden:
 
 api
+diagram
 auto_examples/index
 contributing
 authors