docs(releases): clean up release index with Python compat notes

- Remove redundant intro paragraph
- Add seealso linking to upgrade guide
- Add Python compatibility notes per series (3.x: 3.9+, 2.x: last
  with 3.7/3.8, 1.x: last with Python 2.x)
- Standardize section heading levels and format

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: fgmacedo

docs/releases/index.md

@@ -5,12 +5,13 @@ Versions follow [Semantic Versioning](https://semver.org/) (`<major>.<minor>.<pa
 Backward incompatible (breaking) changes will only be introduced in major versions
 with advance notice in the **Deprecations** section of releases.
 
+```{seealso}
+Upgrading from 2.x? See [](upgrade_2x_to_3.md) for a step-by-step migration guide.
+```
 
-## Releases
-
-Below are release notes through StateMachine and its patch releases.
+## 3.x releases
 
-###  3.* releases
+Requires Python 3.9+.
 
 ```{toctree}
 :maxdepth: 2
@@ -19,7 +20,9 @@ Below are release notes through StateMachine and its patch releases.
 
 ```
 
-###  2.* releases
+## 2.x releases
+
+Last series to support Python 3.7 and 3.8.
 
 ```{toctree}
 :maxdepth: 2
@@ -43,9 +46,9 @@ Below are release notes through StateMachine and its patch releases.
 ```
 
 
-###  1.* releases
+## 1.x releases
 
-This is the last release series to support Python 2.X series.
+Last series to support Python 2.x.
 
 ```{toctree}
 :maxdepth: 2
@@ -57,7 +60,7 @@ This is the last release series to support Python 2.X series.
 
 ```
 
-###  0.* releases
+## 0.x releases
 
 ```{toctree}
 :maxdepth: 1