Add Available Versions to Pulumi CLI navigation

Adds the Pulumi CLI Versions page to the Infrastructure as Code > Pulumi CLI
section in the navigation menu as "Available versions", addressing user
feedback that this important reference page was not easily discoverable.

The page appears after Commands, Environment variables, and Command-line
completion entries, maintaining a logical navigation hierarchy.

Fixes #16370

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: jkodroff

content/docs/get-started/download-install/versions.md

@@ -4,6 +4,10 @@ meta_desc: This page provides an list of available versions of the Pulumi CLI.
 menu:
   install:
     weight: 1
+  iac:
+    name: Available versions
+    weight: 4
+    parent: iac-cli
 
 aliases:
 - /docs/iac/download-install/versions/