Update handbook calendly instructions and fix some handbook grammar.
Apologies for bunching some unrelated changes together in one merge request. The other changes were so minor that they don't seem to warrant separate branches/MRs.
This is mainly to clarify some things in the Calendly section.
- make adding your Calendly to your Slack profile more than a suggestion.
- ensure that the hangout link makes it into the description of all event types (not just the 45min one)
- remind team members to use Calendly, not Google Calendar to cancel or modify an event that was created by Calendly.
Related onboarding issue: https://dev.gitlab.org/gitlab/organization/issues/948