docs: point both alpha and testnet developer docs at v4.2.0-aztecnr-rc.2

Instead of maintaining separate v4.1.3 developer docs for testnet,
both alpha and testnet now share v4.2.0-aztecnr-rc.2. This matches
the operator docs pattern where both networks show a single combined
'Alpha / Testnet' version in the dropdown.

- Removed developer_versioned_docs/version-v4.1.3/ entirely
- Updated developer_version_config.json: testnet → v4.2.0-aztecnr-rc.2
- developer_versions.json auto-generated with single entry

Author: AztecBot

docs/developer_version_config.json

@@ -1,4 +1,4 @@
 {
   "mainnet": "v4.2.0-aztecnr-rc.2",
-  "testnet": "v4.1.3"
+  "testnet": "v4.2.0-aztecnr-rc.2"
 }