Add Verify Stage development guide to navigation

Add Verify Stage development guide (https://docs.gitlab.com/ee/development/contributing/verify/) to the navigation.

This page was discussed in gitlab#344533 (comment 940156988). The page is not in the docs navigation, making it difficult to discover.

Having it next to CI/CD development guide makes it easier for anyone who wants to contribute to Verify and CI/CD code to discover the guide.