Add a top level Support page

Please review and add comments / proposed edits @amara @ChadMalchow @stanhu @sytses @marcia

• Closes https://gitlab.com/gitlab-com/support/issues/186
• Closes https://gitlab.com/gitlab-com/customer-success/issues/12
• Related https://gitlab.com/gitlab-com/support/issues/178

\cc @gl-support