Add new skills for marketing and user engagement

Commit notes

- Introduced multiple new skills including copy-editing, copywriting, email-sequence, form-cro, launch-strategy, marketing-ideas, marketing-psychology, onboarding-cro, popup-cro, pricing-strategy, programmatic-seo, referral-program, and signup-flow-cro. - Each skill includes detailed descriptions, core principles, and practical guidance to enhance user experience and marketing effectiveness. - These additions aim to provide comprehensive resources for users looking to optimize their marketing strategies and user engagement processes.