Update Pagerduty integration guide

[calebhailey]

Our partner page on pagerduty.com links to documentation for installing the old Sensu Enterprise "Classic" integration. We need to update this with instructions on how to install and configure the new Sensu Go integration.

See:

• https://www.pagerduty.com/integrations/sensu/
• https://www.pagerduty.com/docs/guides/sensu-integration-guide/ (these are the old docs)

[calebhailey]

For Team Sensu: I have some additional details I can share via email for next steps here.