Update SIS Guidebook Page to remove SP2013 mentions & update support process

Guidebook Page: https://guidebook.devops.uis.cam.ac.uk/services/sissp/#source-code

1. SIS runs on SharePoint 2019 but the Guidebook page still references SharePoint 2013.

   All SP2013 references should be removed from the Guidebook Page.

2. Replace the <role email> placeholders near top of page with HEAT Ticket process mention (email UIS ServiceDesk).

3. Replace placeholders at page bottom for Service Manager etc. with "TBD".