Update summary of main Contribution Events Initiative page

Problem/Motivation/Idea

The summary on the main initiative page isn't very clear and needs improvement.

Proposed resolution

Rewrite and make clear: 1) what is a contribution event, 2) mission of initiative, and 3) how people can get involved (even if just pointing at existing resources/pages).

Remaining tasks

1. Create draft
2. Review and approve
3. Update documentation