Add handbook link to deprecation issue template

.gitlab/issue_templates/Deprecations.md

+For guidance on the overall deprecations, removals and breaking changes workflow, please visit https://about.gitlab.com/handbook/product/gitlab-the-product/#breaking-changes-deprecations-and-removing-features
+
 <!-- Use this template as a starting point for deprecations. -->
 
 ### Deprecation Summary
@@ -75,4 +77,4 @@ Use the following resources to find the appropriate labels:
 <!-- Identifies that this Issue is related to deprecating a feature -->
 /label ~"type::deprecation"
 
-<!-- Add the ~"breaking change" label to this issue if necessary -->
\ No newline at end of file
+<!-- Add the ~"breaking change" label to this issue if necessary -->