Clarify Calendly conflict resolution with PagerDuty

Why is this change being made?

When following the directions to automatically block your Pagerduty schedule from your Calendly availability, it was unclear to me:

• How is this working (what parts are involved)?
• How to add your Pagerduty WebCal subscription in Google Calendar and why?
• Where to find the Calendar Connections in Calendly?

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files. (this requirement has been removed pending identification of a new DRI for the handbook)