Add Pronovix/ddev-saml-idp to the test IdP setup guide

Problem/Motivation

The current documentation focuses on generic Docker-based approaches for setting up a SAML Identity Provider for local development and testing.

Since DDEV has become the de facto standard local development environment across much of the Drupal ecosystem, it would be beneficial to include the Pronovix/ddev-saml-idp addon as a recommended option for Drupal developers.

The addon provides a ready-to-use SAML Identity Provider that integrates with DDEV and significantly simplifies local SAML testing. In the case of SamlAuth, the addon can be configured with minimal effort and currently eliminates the manual setup steps typically required when configuring a local test IdP.

Steps to reproduce

Proposed resolution