chore: migrate architecture docs

docs/changelog.md

@@ -2,6 +2,10 @@
 
 This changelog documents user-relevant changes to the tmate SSH server charm.
 
+## 2025-12-17
+
+- Moved charm-architecture.md from Explanation to Reference category.
+
 ## 2025-10-28
 
 - Added docs on how to upgrade the charm.

docs/index.md

@@ -40,8 +40,8 @@ Thinking about using the tmate SSH server operator for your next project?
   1. [Upgrade](how-to/upgrade.md)
 1. [Reference](reference)
   1. [Actions](reference/actions.md)
+  1. [Charm architecture](reference/charm-architecture.md)
   1. [Configurations](reference/configurations.md)
   1. [Integrations](reference/integrations.md)
 1. [Explanation](explanation)
   1. [Port Number](explanation/port-number.md)
-  1. [Charm architecture](explanation/charm-architecture.md)