Added Culture Curation mission statement, OKRs, audience, and channels to Corporate Marketing Handbook

Added Culture Curation mission statement, OKRs, audience, and channels to Corporate Marketing Handbook

Given the new nature of this role and its unique ability to touch corporate marketing, employment branding and diversity & inclusion, this section attempts to add transparency about its intent.

By embracing GitLab's Handbook first approach, our team can more easily and expeditiously create an understanding across the organization of the purpose for this role (and, as we grow, this team) within corporate marketing.