Add link to PagerDuty schedules

Why is this change being made?

Adding more documentation surrounding the SRE shadow for new comers to the program so that they are able to accesses schedules and understand the typical activities shadowed.

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what