Docs: Link to DeclarativePolicy framework from development guide

What does this MR do?

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

Author's checklist

• [-] Apply the correct labels and milestone
• [-] Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• [-] Correctly apply the product badges and tiers
• [-] Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• [-] PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• [-] Merge the EE-MR first, CE-MR afterwards