Adds a new governance page covering Slack/mailing list, GitLab registration, expert registration, and step-by-step contribution guide. Adds "Get Involved" CTA block to the landing page with direct links to join.
Closes #32 Closes #35 Closes #38
#32 — "Add section on how to join" — explicitly says "Copy/reference from UNTP" #35 — "Landing page obvious action buttons to join mailing list / join Slack channel" #38 — "Add step-by-step guide on how to add/contribute documents" (came up with Rudy on a call)