Check for removed URLs

Added in version 1.2.0.

The starter pack includes a GitHub action to identify when page URLs have been removed. This includes moving pages to another path, or removing them completely.

This does not cover higher-level changes to URL paths, such as changing the RTD project name, or language and versioning structure provided by RTD.

This check is available to ensure that redirects are implemented when pages are moved, or appropriate information can be provided when anything is removed.