Add link to DeclarativePolicy framework for development guide

What does this MR do?

Doc - link to https://docs.gitlab.com/ee/development/policies.html from https://docs.gitlab.com/ee/development

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides