docs: fix sidebar capitalization and add migration guide

- Add _category_.json for Architecture folder to fix lowercase label
- Move migration guide to top-level docs for sidebar visibility

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: thomhurst

docs/docs/architecture/_category_.json

@@ -0,0 +1,8 @@
+{
+  "label": "Architecture",
+  "position": 6,
+  "link": {
+    "type": "generated-index",
+    "description": "Learn about the architecture of Modular Pipelines."
+  }
+}

docs/docs/how-to/migrating-to-v3.md docs/docs/migrating-to-v3.mddocs/docs/how-to/migrating-to-v3.md renamed to docs/docs/migrating-to-v3.md

@@ -1,6 +1,6 @@
 ---
 title: Migrating to V3
-sidebar_position: 99
+sidebar_position: 7
 ---
 
 # Migrating from V2 to V3