Add PDK.markdown stub so {{pdk}}/pdk.html links resolve

Five docs pages define [pdk]: {{pdk}}/pdk.html as a link reference.
Since {{pdk}} resolves to an empty string, all these links pointed to
/pdk.html which 404'd. Adds PDK.markdown at the repo root with
permalink: /pdk.html, noting that Puppet/Perforce ceased open-source
PDK maintenance at version 3.4.0 and linking to both the OSS docs and
Puppet's current (credential-gated) PDK documentation.

Signed-off-by: Michael Harp <mike@mikeharp.com>
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: miharp

PDK.markdown

@@ -0,0 +1,22 @@
+---
+layout: default
+title: Puppet Development Kit (PDK)
+permalink: /pdk.html
+---
+
+The **Puppet Development Kit (PDK)** is a package of development and testing tools that helps you create, validate, and test Puppet modules.
+
+> **Note:** Puppet (Perforce) ceased open-source maintenance of PDK at version **3.4.0**. Newer versions are distributed from a protected repository requiring either a Puppet Forge API key or a Puppet Enterprise license ID to download; see [Puppet's PDK documentation](https://help.puppet.com/pdk/current/topics/pdk.htm) for details. Documentation for the last open-source release is available in the [pdk repository on GitHub](https://github.com/puppetlabs/pdk/blob/main/docs/pdk.md).
+
+## Overview
+
+PDK provides a standard set of tools and workflows for developing Puppet modules, including:
+
+- Module scaffolding and structure validation
+- Unit testing with rspec-puppet
+- Acceptance testing with Litmus
+- Metadata validation
+
+## Compatibility with OpenVox
+
+OpenVox is compatible with PDK **3.4.0** and earlier for module development workflows. The last open-source release can be found in the [pdk repository on GitHub](https://github.com/puppetlabs/pdk).