Added PR Requests for Announcements section to Handbook

Why is this change being made?

Per announcement strategy discussions with corp comms/PR, blog editorial and social media teams, we decided it would be helpful to describe different tiers of announcements and support/activities that would go with each to the PR section of the handbook. This brings more transparency to how we make our decisions on announcement support and also provides a new issue template for team members to make requests.

Closes https://gitlab.com/gitlab-com/marketing/corporate-marketing/issues/1718

Does this MR meet the acceptance criteria? YES

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what