Add a sandbox option for triage-ops

Context

#1187 (closed)

What does this MR do and why?

• Adds a way to test a new processor with the sandbox flag (explained in the docs in this MR)
• Add documentation on how to use this new way of testing processors in all the relevant places

Expected impact & dry-runs

These are strongly recommended to assist reviewers and reduce the time to merge your change.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/tree/master/doc/scheduled#testing-with-a-dry-run on how to perform dry-runs.

Action items

• (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 => @gl-quality/eng-prod
  • Relevant Slack channels =>
  • Engineering week-in-review