Reorganize the scheduled operations docs

What does this MR do and why?

I'm always struggling a bit when getting into the scheduled part of triage-ops.

This MR is an attempt to clarify some concepts, by reorganizing the docs in a different way.

see the rendered markdown

• We now have Schedules and Policies sections. In each section, we explain their purpose, how to create new ones, and how to test them with a dry-run.
• We also explain special kinds of policies, namely generated policies and one-off policies

Expected impact & dry-runs

None. Doc changes only.

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review