docs(releases): separate Releases section, promote upgrade guide

- Create dedicated "Releases" toctree section in main index
- Move upgrade_2x_to_3 from releases/index to main Releases section
- Add seealso link to upgrade guide at top of 3.0.0 release notes

Author: fgmacedo

docs/index.md

@@ -51,11 +51,18 @@ weighted_transitions
 :caption: Reference
 :maxdepth: 2
 
-releases/3.0.0
 api
 auto_examples/index
 contributing
 authors
+```
+
+```{toctree}
+:caption: Releases
+:maxdepth: 2
+
+releases/3.0.0
+releases/upgrade_2x_to_3
 releases/index
 ```
 

docs/releases/3.0.0.md

@@ -2,6 +2,11 @@
 
 *Not released yet*
 
+```{seealso}
+Upgrading from 2.x? See [](upgrade_2x_to_3.md) for a step-by-step
+migration guide.
+```
+
 ## What's new in 3.0.0
 
 Statecharts are there! 🎉

docs/releases/index.md

@@ -16,7 +16,6 @@ Below are release notes through StateMachine and its patch releases.
 :maxdepth: 2
 
 3.0.0
-upgrade_2x_to_3
 
 ```