update API doc with +API (h1 heading)

doc/api/access_requests.md

doc/api/enviroments.md

doc/api/groups.md

doc/api/issues.md

doc/api/keys.md

doc/api/labels.md

doc/api/members.md

doc/api/merge_requests.md

doc/api/notification_settings.md

doc/api/projects.md

doc/api/settings.md

doc/api/snippets.md

doc/api/todos.md

doc/api/users.md