Add a new "Guide To Monitor" with walkthroughs of the new editor.
Updates from review of monitoring guide.
Updates to create monitor docs based on feedback.
Convert monitor guide to markdown.
More updates of monitoring guide.

- Add info about API to FAQ.
- Add info about notifying users outside of datadog.
Add monitor API docs.

- Document different monitor types.
- Document all available options for monitors, including downtime
  and scoped muting.
Fix up API endpoints for monitors.
Merge branch 'conor/guide-to-monitoring' into staging
Merge branch 'conor/monitor-api' into staging